feat: infinite scroll for folder view

Replace page buttons with IntersectionObserver sentinel — scrolling near
the bottom automatically fetches and appends the next page of results.
Shows a loading indicator while fetching and a total count footer when
all emails are loaded.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/FolderView.jsx

@@ -1,4 +1,4 @@
-import { useEffect, useState, useCallback } from 'react';
+import { useEffect, useState, useCallback, useRef } from 'react';
 import { useParams } from 'react-router-dom';
 import { SearchApi } from '../api/client.js';
 
@@ -50,73 +50,101 @@ export default function FolderView() {
   const { slug } = useParams();
   const meta = FOLDER_META[slug] ?? { icon: '📁', label: slug };
 
-  const [result, setResult] = useState(null);
+  const [emails, setEmails] = useState([]);
+  const [totalCount, setTotalCount] = useState(null);
   const [page, setPage] = useState(1);
+  const [hasMore, setHasMore] = useState(true);
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState(null);
 
-  const load = useCallback((p) => {
-    setLoading(true);
+  // Reset when the folder changes
+  useEffect(() => {
+    setEmails([]);
+    setTotalCount(null);
+    setPage(1);
+    setHasMore(true);
     setError(null);
+  }, [slug]);
+
+  // Fetch a page and append results
+  const fetchPage = useCallback((p) => {
+    setLoading(true);
     SearchApi.folder(slug, p, PAGE_SIZE)
-      .then((r) => { setResult(r); setPage(p); })
+      .then((r) => {
+        setEmails((prev) => p === 1 ? r.items : [...prev, ...r.items]);
+        setTotalCount(r.totalCount);
+        setHasMore(p < r.totalPages);
+        setPage(p);
+      })
       .catch(() => setError('Failed to load emails.'))
       .finally(() => setLoading(false));
   }, [slug]);
 
-  useEffect(() => { load(1); }, [load]);
+  // Initial load
+  useEffect(() => { fetchPage(1); }, [fetchPage]);
 
-  const emails = result?.items ?? [];
-  const totalPages = result?.totalPages ?? 1;
+  // Sentinel div observed to trigger next page
+  const sentinelRef = useRef(null);
+  useEffect(() => {
+    const el = sentinelRef.current;
+    if (!el) return;
+    const observer = new IntersectionObserver(
+      (entries) => {
+        if (entries[0].isIntersecting && hasMore && !loading) {
+          fetchPage(page + 1);
+        }
+      },
+      { rootMargin: '200px' }
+    );
+    observer.observe(el);
+    return () => observer.disconnect();
+  }, [hasMore, loading, page, fetchPage]);
 
   return (
     <div className="folder-view">
       <div className="folder-view-head">
         <h2 className="folder-view-title">{meta.icon} {meta.label}</h2>
-        {result && (
-          <span className="folder-view-count">{result.totalCount.toLocaleString()} email{result.totalCount !== 1 ? 's' : ''}</span>
+        {totalCount !== null && (
+          <span className="folder-view-count">{totalCount.toLocaleString()} email{totalCount !== 1 ? 's' : ''}</span>
         )}
       </div>
 
-      {loading && <div className="fv-loading">Loading…</div>}
       {error && <div className="fv-error">{error}</div>}
 
       {!loading && !error && emails.length === 0 && (
         <div className="fv-empty">No emails in this folder.</div>
       )}
 
-      {!loading && emails.length > 0 && (
-        <>
-          <table className="email-list">
-            <tbody>
-              {emails.map((e) => (
-                <tr key={e.id} className={`email-row${e.isUnread ? ' email-row--unread' : ''}`}>
-                  <td className="el-unread">{e.isUnread && <span className="unread-dot" />}</td>
-                  <td className="el-sender" title={e.senderAddress}>
-                    {e.senderDisplayName || e.senderAddress}
-                  </td>
-                  <td className="el-subject">
-                    <span className="el-subj-text">{e.subject || '(no subject)'}</span>
-                    {e.snippet && <span className="el-snippet"> — {e.snippet}</span>}
-                  </td>
-                  <td className="el-meta">
-                    {e.hasAttachments && <span className="el-attach" title="Has attachment">📎</span>}
-                    {e.sizeEstimateBytes > 1048576 && <span className="el-size">{fmtSize(e.sizeEstimateBytes)}</span>}
-                  </td>
-                  <td className="el-date">{fmtDate(e.sentAtUtc)}</td>
-                </tr>
-              ))}
-            </tbody>
-          </table>
+      {emails.length > 0 && (
+        <table className="email-list">
+          <tbody>
+            {emails.map((e) => (
+              <tr key={e.id} className={`email-row${e.isUnread ? ' email-row--unread' : ''}`}>
+                <td className="el-unread">{e.isUnread && <span className="unread-dot" />}</td>
+                <td className="el-sender" title={e.senderAddress}>
+                  {e.senderDisplayName || e.senderAddress}
+                </td>
+                <td className="el-subject">
+                  <span className="el-subj-text">{e.subject || '(no subject)'}</span>
+                  {e.snippet && <span className="el-snippet"> — {e.snippet}</span>}
+                </td>
+                <td className="el-meta">
+                  {e.hasAttachments && <span className="el-attach" title="Has attachment">📎</span>}
+                  {e.sizeEstimateBytes > 1048576 && <span className="el-size">{fmtSize(e.sizeEstimateBytes)}</span>}
+                </td>
+                <td className="el-date">{fmtDate(e.sentAtUtc)}</td>
+              </tr>
+            ))}
+          </tbody>
+        </table>
+      )}
 
-          {totalPages > 1 && (
-            <div className="fv-pagination">
-              <button disabled={page <= 1} onClick={() => load(page - 1)}>← Prev</button>
-              <span>Page {page} of {totalPages}</span>
-              <button disabled={page >= totalPages} onClick={() => load(page + 1)}>Next →</button>
-            </div>
-          )}
-        </>
+      {/* Sentinel — triggers next page load when scrolled into view */}
+      <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()} emails —</div>
       )}
     </div>
   );