feat(ui): split-view reading pane for FolderView + SearchResults

Clicking an email now opens a collapsible, horizontally resizable
reading pane beside the list instead of a new tab. Extracts the shared
EmailDetail component (full detail fetch + lazy AI summary + Open in
Gmail) and adds list skeleton + empty states via the ui primitives.

- New components/EmailDetail.jsx (emailId/email + onClose)
- New split.css (flex master-detail, native resize, <768px overlay)
- FolderView/SearchResults: selectedEmail state, onOpen wiring,
  Skeleton loading rows, EmptyState zero-states
- main.jsx imports split.css

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/EmailDetail.jsx

@@ -0,0 +1,115 @@
+import { useEffect, useState } from 'react';
+import { EmailApi } from '../api/client.js';
+
+// ── helpers ──────────────────────────────────────────────────────────────────
+
+const fmtSize = (b) => {
+  if (!b) return '';
+  if (b < 1024) return `${b} B`;
+  if (b < 1048576) return `${(b / 1024).toFixed(0)} KB`;
+  return `${(b / 1048576).toFixed(1)} MB`;
+};
+
+/**
+ * Shared reading-pane / full single-email view.
+ *
+ * Accepts either an `email` summary object (as emitted by the list rows) or a
+ * bare `emailId`. Fetches the full detail via EmailApi.get(id) and the AI
+ * summary lazily via EmailApi.summary(id). Renders subject, metadata, an AI
+ * summary button, the body, and per-email actions including "Open in Gmail".
+ *
+ * `onClose` collapses the pane.
+ */
+export default function EmailDetail({ email: summary, emailId, onClose }) {
+  const id = summary?.id ?? emailId;
+
+  const [detail, setDetail] = useState(null);
+  const [loading, setLoading] = useState(true);
+  const [aiSummary, setAiSummary] = useState(null);
+  const [aiLoading, setAiLoading] = useState(false);
+
+  useEffect(() => {
+    if (id == null) return;
+    let cancelled = false;
+    setDetail(null);
+    setLoading(true);
+    setAiSummary(null);
+    setAiLoading(false);
+    EmailApi.get(id)
+      .then((d) => { if (!cancelled) setDetail(d); })
+      .catch(() => { if (!cancelled) setDetail(null); })
+      .finally(() => { if (!cancelled) setLoading(false); });
+    return () => { cancelled = true; };
+  }, [id]);
+
+  const email = detail ?? summary ?? {};
+
+  const fetchAiSummary = () => {
+    if (id == null) return;
+    setAiLoading(true);
+    EmailApi.summary(id)
+      .then((r) => setAiSummary(r.summary))
+      .catch(() => setAiSummary(null))
+      .finally(() => setAiLoading(false));
+  };
+
+  const openInGmail = () => window.open(
+    `https://mail.google.com/mail/u/0/#all/${email.gmailMessageId}`,
+    '_blank', 'noopener,noreferrer'
+  );
+
+  return (
+    <div className="sv-detail">
+      <div className="sv-detail-topbar">
+        <button className="sv-close" onClick={onClose} aria-label="Close reading pane" title="Close">
+          ✕
+        </button>
+      </div>
+
+      <div className="sv-detail-header">
+        <div className="sv-detail-subject">{email.subject || '(no subject)'}</div>
+        <div className="sv-detail-meta">
+          <span>{email.senderDisplayName || email.senderAddress}</span>
+          {email.sentAtUtc && (
+            <>
+              <span className="sv-detail-sep">·</span>
+              <span>{new Date(email.sentAtUtc).toLocaleString()}</span>
+            </>
+          )}
+          {email.sizeEstimateBytes > 0 && (
+            <>
+              <span className="sv-detail-sep">·</span>
+              <span>{fmtSize(email.sizeEstimateBytes)}</span>
+            </>
+          )}
+        </div>
+      </div>
+
+      {!loading && (
+        <div className="sv-ai-summary">
+          {aiSummary != null ? (
+            <div className="sv-ai-summary-text">✨ {aiSummary}</div>
+          ) : (
+            <button className="btn-sm" onClick={fetchAiSummary} disabled={aiLoading}>
+              {aiLoading ? 'Summarising…' : '✨ AI summary'}
+            </button>
+          )}
+        </div>
+      )}
+
+      {loading && <div className="sv-body-loading muted">Loading message…</div>}
+
+      {!loading && detail?.bodyText && (
+        <div className="sv-body">{detail.bodyText}</div>
+      )}
+
+      {!loading && !detail?.bodyText && email.snippet && (
+        <div className="sv-snippet">{email.snippet}</div>
+      )}
+
+      <div className="sv-detail-actions">
+        <button className="btn-sm" onClick={openInGmail}>Open in Gmail ↗</button>
+      </div>
+    </div>
+  );
+}

frontend/src/main.jsx

@@ -12,6 +12,7 @@ import Layout from './components/Layout.jsx';
 import { ToastProvider, TooltipProvider } from './components/ui';
 import './index.css';
 import './styles.css';
+import './split.css';
 
 ReactDOM.createRoot(document.getElementById('root')).render(
   <React.StrictMode>

frontend/src/pages/FolderView.jsx

@@ -2,7 +2,9 @@ import { useEffect, useState, useCallback, useRef } from 'react';
 import { useParams } from 'react-router-dom';
 import { SearchApi } from '../api/client.js';
 import EmailRow from '../components/EmailRow.jsx';
+import EmailDetail from '../components/EmailDetail.jsx';
 import BulkToolbar from '../components/BulkToolbar.jsx';
+import { Skeleton, EmptyState } from '../components/ui';
 import useSelection from '../hooks/useSelection.js';
 import useListKeyboardNav from '../hooks/useListKeyboardNav.js';
 
@@ -35,6 +37,20 @@ const FOLDER_META = {
 
 const PAGE_SIZE = 50;
 
+function ListSkeleton({ rows = 6 }) {
+  return (
+    <div className="sv-skeleton-list" aria-hidden="true">
+      {Array.from({ length: rows }).map((_, i) => (
+        <div className="sv-skeleton-row" key={i}>
+          <Skeleton className="h-3 w-3 rounded-full" />
+          <Skeleton className="h-3 flex-1" />
+          <Skeleton className="h-3 w-16" />
+        </div>
+      ))}
+    </div>
+  );
+}
+
 
 export default function FolderView() {
   const { slug } = useParams();
@@ -46,6 +62,7 @@ export default function FolderView() {
   const [hasMore, setHasMore] = useState(true);
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState(null);
+  const [selectedEmail, setSelectedEmail] = useState(null);
   const { selected, toggle, clear, removeIds, selectedIds } = useSelection();
   const focusedId = useListKeyboardNav(emails, (id) => setEmails((prev) => prev.filter((x) => x.id !== id)));
 
@@ -56,6 +73,7 @@ export default function FolderView() {
     setPage(1);
     setHasMore(true);
     setError(null);
+    setSelectedEmail(null);
     clear();
   }, [slug, clear]);
 
@@ -115,34 +133,57 @@ export default function FolderView() {
         }}
       />
 
-      {!loading && !error && emails.length === 0 && (
-        <div className="fv-empty">No emails in this folder.</div>
-      )}
+      <div className={`sv-split${selectedEmail ? ' sv-split--open' : ''}`}>
+        <div className="sv-list-pane">
+          {loading && emails.length === 0 && !error && <ListSkeleton />}
 
-      {emails.length > 0 && (
-        <table className="email-list">
-          <tbody>
-            {emails.map((e) => (
-              <EmailRow
-                key={e.id}
-                email={e}
-                selected={selected.has(e.id)}
-                onToggleSelect={toggle}
-                focused={focusedId === e.id}
-                onRemove={(id) => setEmails((prev) => prev.filter((x) => x.id !== id))}
-              />
-            ))}
-          </tbody>
-        </table>
-      )}
+          {!loading && !error && emails.length === 0 && (
+            <EmptyState
+              title="No emails in this folder"
+              description="Nothing here yet — try another folder or run a sync."
+            />
+          )}
 
-      {/* Sentinel — triggers next page load when scrolled into view */}
-      <div ref={sentinelRef} className="fv-sentinel" />
+          {emails.length > 0 && (
+            <table className="email-list">
+              <tbody>
+                {emails.map((e) => (
+                  <EmailRow
+                    key={e.id}
+                    email={e}
+                    selected={selected.has(e.id)}
+                    onToggleSelect={toggle}
+                    focused={focusedId === e.id}
+                    onOpen={(email) => setSelectedEmail(email)}
+                    onRemove={(id) => {
+                      setEmails((prev) => prev.filter((x) => x.id !== id));
+                      setSelectedEmail((cur) => (cur?.id === id ? null : cur));
+                    }}
+                  />
+                ))}
+              </tbody>
+            </table>
+          )}
 
-      {loading && <div className="fv-loading-more">Loading…</div>}
-      {!hasMore && emails.length > 0 && (
-        <div className="fv-end">— {emails.length.toLocaleString()} emails —</div>
-      )}
+          {/* Sentinel — triggers next page load when scrolled into view */}
+          <div ref={sentinelRef} className="fv-sentinel" />
+
+          {loading && emails.length > 0 && <div className="fv-loading-more">Loading…</div>}
+          {!hasMore && emails.length > 0 && (
+            <div className="fv-end">— {emails.length.toLocaleString()} emails —</div>
+          )}
+        </div>
+
+        {selectedEmail && (
+          <aside className="sv-reading-pane">
+            <EmailDetail
+              key={selectedEmail.id}
+              email={selectedEmail}
+              onClose={() => setSelectedEmail(null)}
+            />
+          </aside>
+        )}
+      </div>
     </div>
   );
 }

frontend/src/pages/SearchResults.jsx

@@ -2,13 +2,29 @@ import { useEffect, useState, useCallback, useRef } from 'react';
 import { useSearchParams } from 'react-router-dom';
 import { SearchApi } from '../api/client.js';
 import EmailRow from '../components/EmailRow.jsx';
+import EmailDetail from '../components/EmailDetail.jsx';
 import BulkToolbar from '../components/BulkToolbar.jsx';
+import { Skeleton, EmptyState } from '../components/ui';
 import useSelection from '../hooks/useSelection.js';
 import useListKeyboardNav from '../hooks/useListKeyboardNav.js';
 import useSavedSearches from '../hooks/useSavedSearches.js';
 
 const PAGE_SIZE = 50;
 
+function ListSkeleton({ rows = 6 }) {
+  return (
+    <div className="sv-skeleton-list" aria-hidden="true">
+      {Array.from({ length: rows }).map((_, i) => (
+        <div className="sv-skeleton-row" key={i}>
+          <Skeleton className="h-3 w-3 rounded-full" />
+          <Skeleton className="h-3 flex-1" />
+          <Skeleton className="h-3 w-16" />
+        </div>
+      ))}
+    </div>
+  );
+}
+
 
 export default function SearchResults() {
   const [searchParams] = useSearchParams();
@@ -20,6 +36,7 @@ export default function SearchResults() {
   const [hasMore, setHasMore] = useState(true);
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState(null);
+  const [selectedEmail, setSelectedEmail] = useState(null);
   const { selected, toggle, clear, removeIds, selectedIds } = useSelection();
   const focusedId = useListKeyboardNav(emails, (id) => setEmails((prev) => prev.filter((x) => x.id !== id)));
   const { searches: savedSearches, add: addSavedSearch } = useSavedSearches();
@@ -36,6 +53,7 @@ export default function SearchResults() {
     setPage(1);
     setHasMore(true);
     setError(null);
+    setSelectedEmail(null);
     clear();
   }, [q, clear]);
 
@@ -83,11 +101,13 @@ export default function SearchResults() {
         )}
       </div>
 
-      {!q.trim() && <div className="fv-empty">Enter a search query above.</div>}
-      {error && <div className="fv-error">{error}</div>}
-      {!loading && !error && q.trim() && emails.length === 0 && !hasMore && (
-        <div className="fv-empty">No results for "{q}".</div>
+      {!q.trim() && (
+        <EmptyState
+          title="Search your mail"
+          description="Enter a search query above to find emails."
+        />
       )}
+      {error && <div className="fv-error">{error}</div>}
 
       <BulkToolbar
         selectedIds={selectedIds}
@@ -100,27 +120,56 @@ export default function SearchResults() {
         }}
       />
 
-      {emails.length > 0 && (
-        <table className="email-list">
-          <tbody>
-            {emails.map((e) => (
-              <EmailRow
-                key={e.id}
-                email={e}
-                selected={selected.has(e.id)}
-                onToggleSelect={toggle}
-                focused={focusedId === e.id}
-                onRemove={(id) => setEmails((prev) => prev.filter((x) => x.id !== id))}
-              />
-            ))}
-          </tbody>
-        </table>
-      )}
+      {q.trim() && (
+        <div className={`sv-split${selectedEmail ? ' sv-split--open' : ''}`}>
+          <div className="sv-list-pane">
+            {loading && emails.length === 0 && !error && <ListSkeleton />}
 
-      <div ref={sentinelRef} className="fv-sentinel" />
-      {loading && <div className="fv-loading-more">Loading…</div>}
-      {!hasMore && emails.length > 0 && (
-        <div className="fv-end">— {emails.length.toLocaleString()} results —</div>
+            {!loading && !error && emails.length === 0 && !hasMore && (
+              <EmptyState
+                title={`No results for "${q}"`}
+                description="Try a different search term or filter."
+              />
+            )}
+
+            {emails.length > 0 && (
+              <table className="email-list">
+                <tbody>
+                  {emails.map((e) => (
+                    <EmailRow
+                      key={e.id}
+                      email={e}
+                      selected={selected.has(e.id)}
+                      onToggleSelect={toggle}
+                      focused={focusedId === e.id}
+                      onOpen={(email) => setSelectedEmail(email)}
+                      onRemove={(id) => {
+                        setEmails((prev) => prev.filter((x) => x.id !== id));
+                        setSelectedEmail((cur) => (cur?.id === id ? null : cur));
+                      }}
+                    />
+                  ))}
+                </tbody>
+              </table>
+            )}
+
+            <div ref={sentinelRef} className="fv-sentinel" />
+            {loading && emails.length > 0 && <div className="fv-loading-more">Loading…</div>}
+            {!hasMore && emails.length > 0 && (
+              <div className="fv-end">— {emails.length.toLocaleString()} results —</div>
+            )}
+          </div>
+
+          {selectedEmail && (
+            <aside className="sv-reading-pane">
+              <EmailDetail
+                key={selectedEmail.id}
+                email={selectedEmail}
+                onClose={() => setSelectedEmail(null)}
+              />
+            </aside>
+          )}
+        </div>
       )}
     </div>
   );

frontend/src/split.css

@@ -0,0 +1,148 @@
+/* ── Split-view reading pane ────────────────────────────────────────────────
+ * Owned by Unit 1. Gmail/Outlook-style master-detail: the email list stays on
+ * the left, a collapsible + horizontally resizable reading pane sits on the
+ * right. Below 768px the pane overlays the list (mobile stack) instead of
+ * squishing the columns side-by-side.
+ *
+ * These pages (.folder-view) are styled from styles.css, so we reference its
+ * legacy CSS variables (--panel, --panel-2, --text, --muted, --accent) with
+ * hardcoded hex fallbacks. The modern index.css tokens are stored as raw HSL
+ * channel triplets and only work via hsl(var(--x)), so they are NOT used bare
+ * here.
+ * ------------------------------------------------------------------------- */
+
+.sv-split {
+  display: flex;
+  align-items: stretch;
+  gap: 0;
+  width: 100%;
+}
+
+/* Left column — the list. Flexes to fill remaining space and scrolls itself. */
+.sv-list-pane {
+  flex: 1 1 auto;
+  min-width: 0;
+  overflow-x: auto;
+}
+
+/* Right column — the reading pane. Resizable via the native handle; the width
+ * gives it a sensible default, and the user can drag the bottom-right corner.
+ * `resize: horizontal` needs `overflow` other than visible. */
+.sv-reading-pane {
+  flex: 0 0 auto;
+  width: clamp(320px, 42%, 640px);
+  min-width: 300px;
+  max-width: 80vw;
+  border-left: 1px solid var(--panel-2, #222a3d);
+  overflow: auto;
+  resize: horizontal;
+  background: var(--panel, #1a2030);
+  align-self: stretch;
+}
+
+/* ── Reading pane inner content ─────────────────────────────────────────── */
+.sv-detail { padding: 18px 20px; }
+
+.sv-detail-topbar {
+  display: flex;
+  justify-content: flex-end;
+  margin-bottom: 6px;
+}
+.sv-close {
+  background: transparent;
+  border: 1px solid var(--panel-2, #222a3d);
+  color: var(--muted, #8b93a7);
+  border-radius: 6px;
+  width: 28px;
+  height: 28px;
+  font-size: 13px;
+  line-height: 1;
+  cursor: pointer;
+}
+.sv-close:hover {
+  background: var(--panel-2, #222a3d);
+  color: var(--text, #e6e9f0);
+}
+
+.sv-detail-header { margin-bottom: 14px; }
+.sv-detail-subject {
+  font-size: 18px;
+  font-weight: 700;
+  margin-bottom: 6px;
+  color: var(--text, #e6e9f0);
+  word-break: break-word;
+}
+.sv-detail-meta {
+  font-size: 12px;
+  color: var(--muted, #8b93a7);
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px;
+  align-items: center;
+}
+.sv-detail-sep { opacity: 0.4; }
+
+.sv-ai-summary { margin-bottom: 14px; }
+.sv-ai-summary-text {
+  background: var(--panel-2, #222a3d);
+  border: 1px solid var(--panel-2, #222a3d);
+  border-radius: 8px;
+  padding: 10px 14px;
+  font-size: 13px;
+  color: var(--text, #e6e9f0);
+}
+
+.sv-body-loading { padding: 8px 0 18px; }
+.sv-body {
+  background: var(--panel, #1a2030);
+  border: 1px solid var(--panel-2, #222a3d);
+  border-radius: 8px;
+  padding: 16px 18px;
+  font-size: 13px;
+  line-height: 1.7;
+  color: var(--text, #e6e9f0);
+  white-space: pre-wrap;
+  word-break: break-word;
+  margin-bottom: 18px;
+  max-height: 60vh;
+  overflow-y: auto;
+}
+.sv-snippet {
+  font-size: 13px;
+  line-height: 1.6;
+  color: var(--muted, #8b93a7);
+  margin-bottom: 18px;
+}
+.sv-detail-actions { display: flex; gap: 10px; flex-wrap: wrap; }
+
+/* ── List loading / empty states ───────────────────────────────────────── */
+.sv-skeleton-list {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  padding: 12px 0;
+}
+.sv-skeleton-row {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  padding: 4px 8px;
+}
+
+/* ── Mobile: stack / overlay the reading pane ──────────────────────────── */
+@media (max-width: 767px) {
+  .sv-reading-pane {
+    position: fixed;
+    inset: 56px 0 0 0; /* below the topbar */
+    width: 100% !important;
+    max-width: 100vw;
+    min-width: 0;
+    border-left: none;
+    resize: none;
+    z-index: 40;
+  }
+  /* When the pane is open, hide the underlying list to avoid double-scroll. */
+  .sv-split.sv-split--open .sv-list-pane {
+    display: none;
+  }
+}