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}