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/utils/a11y.ts

@@ -0,0 +1,14 @@
+import { timeDayMonthYear, timeHourMinute } from './time';
+
+/**
+ * Builds a plain-text accessible label for a message row, used when the
+ * visible sender/timestamp header is collapsed and therefore hidden from
+ * assistive technology.
+ *
+ * @param sender - Sender display name (already resolved to a human string).
+ * @param ts - Message origin timestamp in milliseconds.
+ * @param hour24Clock - Whether to format the time using a 24-hour clock.
+ * @returns A label such as `Alice, 1 July 2026 14:30`.
+ */
+export const messageAriaLabel = (sender: string, ts: number, hour24Clock: boolean): string =>
+  `${sender}, ${timeDayMonthYear(ts)} ${timeHourMinute(ts, hour24Clock)}`;