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/hooks/useListKeyboardNav.js

@@ -0,0 +1,48 @@
+import { useEffect, useState } from 'react';
+import { BulkApi } from '../api/client.js';
+
+/// <summary>
+/// j/k move focus down/up the list, e archives the focused email, # (shift+3)
+/// trashes it. Ignored while an input/textarea/select has focus, or while
+/// the "/" search shortcut is active, so typing is never hijacked.
+/// `onRemoved(id)` lets the caller drop the row from local state after a
+/// successful archive/trash.
+/// </summary>
+export default function useListKeyboardNav(emails, onRemoved) {
+  const [focusedId, setFocusedId] = useState(null);
+
+  useEffect(() => {
+    const handler = async (e) => {
+      const tag = document.activeElement?.tagName;
+      if (tag === 'INPUT' || tag === 'TEXTAREA' || tag === 'SELECT') return;
+      if (!emails.length) return;
+
+      const idx = emails.findIndex((x) => x.id === focusedId);
+
+      if (e.key === 'j') {
+        e.preventDefault();
+        const next = idx < 0 ? 0 : Math.min(idx + 1, emails.length - 1);
+        setFocusedId(emails[next].id);
+      } else if (e.key === 'k') {
+        e.preventDefault();
+        const prev = idx < 0 ? 0 : Math.max(idx - 1, 0);
+        setFocusedId(emails[prev].id);
+      } else if (e.key === 'e' && idx >= 0) {
+        e.preventDefault();
+        const id = emails[idx].id;
+        await BulkApi.archive([id]);
+        onRemoved(id);
+      } else if (e.key === '#' && idx >= 0) {
+        e.preventDefault();
+        const id = emails[idx].id;
+        await BulkApi.trash([id]);
+        onRemoved(id);
+      }
+    };
+
+    window.addEventListener('keydown', handler);
+    return () => window.removeEventListener('keydown', handler);
+  }, [emails, focusedId, onRemoved]);
+
+  return focusedId;
+}