fix(a11y): review-wave fixes (P3-4)

- `?` shortcut now stopImmediatePropagation so RoomView's type-to-focus handler
  doesn't steal focus into the composer behind the dialog (and swallow Escape) —
  CONFIRMED review finding.
- Typing live region stays mounted (empty when idle) so the FIRST "X is typing"
  is reliably announced (a status region added with its text isn't always read).
- Removed a stray empty `{}` JSX expression in MediaGallery (leftover from an
  auto-fix).

Reviewer verified the rest: collapsed-message labels, focus-return
classification (4 dialogs fixed, popouts correctly left), and all aria fixes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/shortcuts/KeyboardShortcutsDialog.tsx

@@ -55,6 +55,10 @@ export function useKeyboardShortcutsTrigger() {
         // `?` is produced by Shift + `/` on the common layouts.
         if (evt.key === '?') {
           evt.preventDefault();
+          // Stop RoomView's window-level "type any char → focus composer"
+          // handler from also firing — otherwise focus lands in the composer
+          // behind the dialog and Escape gets swallowed by the contenteditable.
+          evt.stopImmediatePropagation();
           setOpen(true);
         }
       },