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/components/EmailRow.jsx

@@ -16,7 +16,7 @@ const fmtSize = (b) => {
   return `${(b / 1048576).toFixed(1)} MB`;
 };
 
-export default function EmailRow({ email: initial, onRemove, selected, onToggleSelect }) {
+export default function EmailRow({ email: initial, onRemove, selected, onToggleSelect, focused }) {
   const [email, setEmail] = useState(initial);
   const [acting, setActing] = useState(false);
 
@@ -68,7 +68,7 @@ export default function EmailRow({ email: initial, onRemove, selected, onToggleS
 
   return (
     <tr
-      className={`email-row${email.isUnread ? ' email-row--unread' : ''}${acting ? ' email-row--acting' : ''}`}
+      className={`email-row${email.isUnread ? ' email-row--unread' : ''}${acting ? ' email-row--acting' : ''}${focused ? ' email-row--focused' : ''}`}
       onClick={openInGmail}
       title="Open in Gmail"
     >