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/room/MediaGallery.tsx

@@ -261,7 +261,6 @@ function Lightbox({
           escapeDeactivates: false,
         }}
       >
-        {}
         <div
           role="dialog"
           aria-modal