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/RoomViewTyping.css.ts

@@ -25,3 +25,16 @@ export const RoomViewTyping = style([
 export const TypingText = style({
   flexGrow: 1,
 });
+
+// Visually hidden but available to assistive technology.
+export const SrOnly = style({
+  position: 'absolute',
+  width: 1,
+  height: 1,
+  padding: 0,
+  margin: -1,
+  overflow: 'hidden',
+  clip: 'rect(0, 0, 0, 0)',
+  whiteSpace: 'nowrap',
+  border: 0,
+});