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/features/room/message/Reactions.tsx

@@ -106,7 +106,6 @@ export const Reactions = as<'div', ReactionsProps>(
               <FocusTrap
                 focusTrapOptions={{
                   initialFocus: false,
-                  returnFocusOnDeactivate: false,
                   onDeactivate: () => setViewer(false),
                   clickOutsideDeactivates: true,
                   escapeDeactivates: stopPropagation,