feat(a11y): focus return, typing announcement, shortcuts help (P3-4)

- Focus returns to the trigger when closing 4 genuine dialogs (room-topic
  viewer, reaction viewer, header topic, Search) — 20 inline popouts/menus
  correctly left as-is (returning focus to a hover target would be wrong).
- Typing indicator announced via a visually-hidden role="status" region;
  the visual text is aria-hidden to avoid double announcement.
- New keyboard-shortcuts help dialog (press ?, ignored while typing),
  mounted in ClientNonUIFeatures.

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

src/app/components/room-intro/RoomIntro.tsx

@@ -95,7 +95,6 @@ export const RoomIntro = as<'div', RoomIntroProps>(({ room, ...props }, ref) =>
                     <FocusTrap
                       focusTrapOptions={{
                         initialFocus: false,
-                        returnFocusOnDeactivate: false,
                         clickOutsideDeactivates: true,
                         onDeactivate: () => setViewTopic(false),
                         escapeDeactivates: stopPropagation,