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.tsx

@@ -37,6 +37,20 @@ export const RoomViewTyping = as<'div', RoomViewTypingProps>(
       return null;
     }
 
+    // A single, non-truncated string for assistive technology to announce.
+    let typingAnnouncement = '';
+    if (typingNames.length === 1) {
+      typingAnnouncement = `${typingNames[0]} is typing`;
+    } else if (typingNames.length === 2) {
+      typingAnnouncement = `${typingNames[0]} and ${typingNames[1]} are typing`;
+    } else if (typingNames.length === 3) {
+      typingAnnouncement = `${typingNames[0]}, ${typingNames[1]} and ${typingNames[2]} are typing`;
+    } else {
+      typingAnnouncement = `${typingNames[0]}, ${typingNames[1]}, ${typingNames[2]} and ${
+        typingNames.length - 3
+      } others are typing`;
+    }
+
     const handleDropAll = () => {
       // some homeserver does not timeout typing status
       // we have given option so user can drop their typing status
@@ -50,7 +64,10 @@ export const RoomViewTyping = as<'div', RoomViewTypingProps>(
     };
 
     return (
-      <div style={{ position: 'relative' }} aria-live="polite" aria-atomic="false">
+      <div style={{ position: 'relative' }}>
+        <span className={css.SrOnly} role="status" aria-live="polite" aria-atomic="true">
+          {typingAnnouncement}
+        </span>
         <Box
           className={classNames(css.RoomViewTyping, className)}
           alignItems="Center"
@@ -59,7 +76,7 @@ export const RoomViewTyping = as<'div', RoomViewTypingProps>(
           ref={ref}
         >
           <TypingIndicator />
-          <Text className={css.TypingText} size="T300" truncate>
+          <Text className={css.TypingText} size="T300" truncate aria-hidden>
             {typingNames.length === 1 && (
               <>
                 <b>{typingNames[0]}</b>