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/pages/client/ClientNonUIFeatures.tsx

@@ -42,6 +42,7 @@ import { toastQueueAtom } from '../../state/toast';
 import { useReminders } from '../../hooks/useReminders';
 import { useTauriUpdater } from '../../hooks/useTauriUpdater';
 import { TauriDesktopFeatures } from '../../components/TauriDesktopFeatures';
+import { KeyboardShortcutsDialog, useKeyboardShortcutsTrigger } from '../../features/shortcuts';
 import { useRoomsListener } from '../../hooks/useRoomsListener';
 import { threadNotificationsAtom } from '../../state/threadNotifications';
 import { roomIdToActiveThreadIdAtomFamily } from '../../state/room/thread';
@@ -213,7 +214,7 @@ function InviteNotifications() {
   ]);
 
   return (
-    <audio ref={audioRef} style={{ display: 'none' }}>
+    <audio ref={audioRef} style={{ display: 'none' }} aria-hidden="true">
       <source src={soundSrc ?? InviteSound} type="audio/ogg" />
     </audio>
   );
@@ -496,7 +497,7 @@ function MessageNotifications() {
   useRoomsListener(mx, ThreadEvent.NewReply, handleNewReply);
 
   return (
-    <audio ref={audioRef} style={{ display: 'none' }}>
+    <audio ref={audioRef} style={{ display: 'none' }} aria-hidden="true">
       <source src={soundSrc ?? NotificationSound} type="audio/ogg" />
     </audio>
   );
@@ -642,6 +643,13 @@ function LotusDenoiseFeature() {
   return null;
 }
 
+// Registers the global `?` shortcut (ignored while typing) and renders the
+// keyboard-shortcuts help dialog. Headless — the dialog self-gates on its atom.
+function KeyboardShortcutsFeature() {
+  useKeyboardShortcutsTrigger();
+  return <KeyboardShortcutsDialog />;
+}
+
 export function ClientNonUIFeatures({ children }: ClientNonUIFeaturesProps) {
   return (
     <>
@@ -656,6 +664,7 @@ export function ClientNonUIFeatures({ children }: ClientNonUIFeaturesProps) {
       <TauriDesktopFeatures />
       <LotusDenoiseFeature />
       <DeepLinkNavigator />
+      <KeyboardShortcutsFeature />
       {children}
     </>
   );