feat(a11y): message semantics for screen readers (P3-4)

- Each message is role="article"; collapsed messages (consecutive from one
  sender) now carry an aria-label with sender + time — previously a screen
  reader heard only the body with no attribution (the biggest a11y gap).
  Pure messageAriaLabel() reuses the existing time utils (+3 tests).
- Editing a message announces "Editing message from <sender>" (ariaLabel
  threaded MessageEditor → CustomEditor; the main composer is unaffected).
- System emoji get role="img" + aria-label from the shortcode; custom
  emoticons always have an accessible name.

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

src/app/features/room/message/Message.tsx

@@ -56,6 +56,7 @@ import {
   getMemberDisplayName,
 } from '../../../utils/room';
 import { getMxIdLocalPart, mxcUrlToHttp } from '../../../utils/matrix';
+import { messageAriaLabel } from '../../../utils/a11y';
 import { MessageLayout, MessageSpacing } from '../../../state/settings';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { useRecentEmoji } from '../../../hooks/useRecentEmoji';
@@ -972,6 +973,10 @@ export const Message = React.memo(
             [MsgAppearClass]: playAppear,
             [MentionHighlightPulse]: playMentionPulse,
           })}
+          role="article"
+          aria-label={
+            collapse ? messageAriaLabel(senderDisplayName, mEvent.getTs(), hour24Clock) : undefined
+          }
           tabIndex={0}
           space={messageSpacing}
           collapse={collapse}

src/app/features/room/message/MessageEditor.tsx

@@ -51,7 +51,13 @@ import { UseStateProvider } from '../../../components/UseStateProvider';
 import { EmojiBoard } from '../../../components/emoji-board';
 import { AsyncStatus, useAsyncCallback } from '../../../hooks/useAsyncCallback';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
-import { getEditedEvent, getMentionContent, trimReplyFromFormattedBody } from '../../../utils/room';
+import {
+  getEditedEvent,
+  getMemberDisplayName,
+  getMentionContent,
+  trimReplyFromFormattedBody,
+} from '../../../utils/room';
+import { getMxIdLocalPart } from '../../../utils/matrix';
 import { mobileOrTablet } from '../../../utils/user-agent';
 import { useComposingCheck } from '../../../hooks/useComposingCheck';
 
@@ -66,6 +72,12 @@ export const MessageEditor = as<'div', MessageEditorProps>(
   ({ room, roomId, mEvent, imagePackRooms, onCancel, ...props }, ref) => {
     const mx = useMatrixClient();
     const editor = useEditor();
+    // Accessible name for the edit textbox so screen readers announce which
+    // message is being edited (a11y, P3-4).
+    const editSenderId = mEvent.getSender();
+    const editSenderName = editSenderId
+      ? (getMemberDisplayName(room, editSenderId) ?? getMxIdLocalPart(editSenderId) ?? editSenderId)
+      : '';
     const [enterForNewline] = useSetting(settingsAtom, 'enterForNewline');
     const [globalToolbar] = useSetting(settingsAtom, 'editorToolbar');
     const [isMarkdown] = useSetting(settingsAtom, 'isMarkdown');
@@ -259,6 +271,7 @@ export const MessageEditor = as<'div', MessageEditorProps>(
         <CustomEditor
           editor={editor}
           placeholder="Edit message..."
+          ariaLabel={editSenderId ? `Editing message from ${editSenderName}` : 'Edit message'}
           onKeyDown={handleKeyDown}
           onKeyUp={handleKeyUp}
           bottom={