feat: bulk actions for email lists

Select multiple rows (checkbox per row) in FolderView, SearchResults,
and the Senders email panel. A toolbar appears with mark read/unread,
star, archive, and trash, applied to the whole selection via
CleanupService.ExecuteAsync (already scoped to UserId).

Shared via useSelection hook and BulkToolbar component to avoid
duplicating selection state across the three list views.

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

frontend/src/pages/Senders.jsx

@@ -1,5 +1,7 @@
 import { useEffect, useCallback, useRef, useState } from 'react';
 import { AnalyticsApi, SearchApi, EmailApi } from '../api/client.js';
+import BulkToolbar from '../components/BulkToolbar.jsx';
+import useSelection from '../hooks/useSelection.js';
 
 // ── helpers ──────────────────────────────────────────────────────────────────
 
@@ -123,7 +125,7 @@ function EmailDetail({ email: summary, onBack }) {
 
 // ── EmailListRow (row inside sender email list) ───────────────────────────────
 
-function EmailListRow({ email: initial, onRemove, onOpen }) {
+function EmailListRow({ email: initial, onRemove, onOpen, selected, onToggleSelect }) {
   const [email, setEmail] = useState(initial);
   const [acting, setActing] = useState(false);
 
@@ -150,6 +152,9 @@ function EmailListRow({ email: initial, onRemove, onOpen }) {
       title="View email"
       style={{ cursor: 'pointer' }}
     >
+      <td className="el-select" onClick={(e) => e.stopPropagation()}>
+        <input type="checkbox" checked={!!selected} onChange={() => onToggleSelect(email.id)} />
+      </td>
       <td className="el-unread">{email.isUnread && <span className="unread-dot" />}</td>
       <td className="el-subject">
         <span className="el-subj-text">{email.subject || '(no subject)'}</span>
@@ -195,6 +200,7 @@ function SenderEmails({ sender }) {
   const [loading, setLoading] = useState(false);
   const [openEmail, setOpenEmail] = useState(null);
   const sentinelRef = useRef(null);
+  const { selected, toggle, clear, removeIds, selectedIds } = useSelection();
 
   // Reset when sender changes
   useEffect(() => {
@@ -203,7 +209,8 @@ function SenderEmails({ sender }) {
     setPage(1);
     setHasMore(true);
     setOpenEmail(null);
-  }, [sender.senderId]);
+    clear();
+  }, [sender.senderId, clear]);
 
   const fetchPage = useCallback((p) => {
     setLoading(true);
@@ -245,6 +252,17 @@ function SenderEmails({ sender }) {
         )}
       </div>
 
+      <BulkToolbar
+        selectedIds={selectedIds}
+        onClear={clear}
+        onDone={(action, ids) => {
+          if (action === 'trash' || action === 'archive') {
+            setEmails((prev) => prev.filter((x) => !ids.includes(x.id)));
+            removeIds(ids);
+          }
+        }}
+      />
+
       {emails.length > 0 && (
         <table className="email-list">
           <tbody>
@@ -252,6 +270,8 @@ function SenderEmails({ sender }) {
               <EmailListRow
                 key={e.id}
                 email={e}
+                selected={selected.has(e.id)}
+                onToggleSelect={toggle}
                 onOpen={setOpenEmail}
                 onRemove={(id) => setEmails((prev) => prev.filter((x) => x.id !== id))}
               />