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/components/EmailRow.jsx

@@ -16,7 +16,7 @@ const fmtSize = (b) => {
   return `${(b / 1048576).toFixed(1)} MB`;
 };
 
-export default function EmailRow({ email: initial, onRemove }) {
+export default function EmailRow({ email: initial, onRemove, selected, onToggleSelect }) {
   const [email, setEmail] = useState(initial);
   const [acting, setActing] = useState(false);
 
@@ -72,6 +72,11 @@ export default function EmailRow({ email: initial, onRemove }) {
       onClick={openInGmail}
       title="Open in Gmail"
     >
+      {onToggleSelect && (
+        <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-sender" title={email.senderAddress}>
         {email.senderDisplayName || email.senderAddress}