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.test.ts

@@ -0,0 +1,28 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import dayjs from 'dayjs';
+import { messageAriaLabel } from './a11y';
+import { timeDayMonthYear, timeHourMinute } from './time';
+
+test('messageAriaLabel composes sender, date and time (24h)', () => {
+  const ts = dayjs('2026-07-01T14:30:00').valueOf();
+  assert.equal(
+    messageAriaLabel('Alice', ts, true),
+    `Alice, ${timeDayMonthYear(ts)} ${timeHourMinute(ts, true)}`,
+  );
+});
+
+test('messageAriaLabel honours the 12-hour clock preference', () => {
+  const ts = dayjs('2026-07-01T14:30:00').valueOf();
+  assert.equal(
+    messageAriaLabel('Bob', ts, false),
+    `Bob, ${timeDayMonthYear(ts)} ${timeHourMinute(ts, false)}`,
+  );
+});
+
+test('messageAriaLabel keeps the sender name verbatim as plain text', () => {
+  const ts = dayjs('2026-07-01T09:05:00').valueOf();
+  const label = messageAriaLabel('@user:example.org', ts, true);
+  assert.ok(label.startsWith('@user:example.org, '));
+  assert.ok(!label.includes('<'));
+});

src/app/utils/a11y.ts

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+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)}`;