feat: keyboard shortcuts for email navigation

'/' focuses the search bar from anywhere (Layout-level listener).
j/k move focus up/down the email list in FolderView and SearchResults;
e archives the focused email, # trashes it. All shortcuts are ignored
while an input/textarea/select has focus, so typing is never hijacked.

A small hint footer in the bottom-right reminds users of the shortcuts.

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

frontend/src/pages/FolderView.jsx

@@ -4,6 +4,7 @@ import { SearchApi } from '../api/client.js';
 import EmailRow from '../components/EmailRow.jsx';
 import BulkToolbar from '../components/BulkToolbar.jsx';
 import useSelection from '../hooks/useSelection.js';
+import useListKeyboardNav from '../hooks/useListKeyboardNav.js';
 
 const FOLDER_META = {
   inbox:      { icon: '📥', label: 'Inbox' },
@@ -46,6 +47,7 @@ export default function FolderView() {
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState(null);
   const { selected, toggle, clear, removeIds, selectedIds } = useSelection();
+  const focusedId = useListKeyboardNav(emails, (id) => setEmails((prev) => prev.filter((x) => x.id !== id)));
 
   // Reset when the folder changes
   useEffect(() => {
@@ -126,6 +128,7 @@ export default function FolderView() {
                 email={e}
                 selected={selected.has(e.id)}
                 onToggleSelect={toggle}
+                focused={focusedId === e.id}
                 onRemove={(id) => setEmails((prev) => prev.filter((x) => x.id !== id))}
               />
             ))}