feat(translation): on-device message translation

Add per-message translation that runs entirely on-device via the
Chromium built-in Translator + LanguageDetector APIs. Message text
never leaves the machine and never touches a cloud service, preserving
the E2EE guarantee. When the on-device engine is unavailable
(non-Chromium / mobile) the feature simply hides itself; there is no
network fallback.

- Engine abstraction (utils/translation): TranslationEngine interface
  plus a chromeTranslationEngine implementation (feature-detected,
  caches translator/detector instances, download-progress monitor).
  Pure lang-code helpers (normalize/sameLanguage/curated targets) with
  unit tests.
- Settings: translateTargetLang (default English) + autoTranslate
  (opt-in), with a Messages settings tile — a target-language select
  and an auto-translate switch, disabled with a note where unsupported.
- useMessageTranslation hook + shared per-event toggle atom-family and a
  persisted LRU cache so scrollback never re-translates.
- UI: a Translate / Show Original message-menu action, an inline
  "Translated from <lang> - Show original" chip, and a body swap in
  m.text/m.emote/m.notice that renders the translated text through the
  plain-text path (linkify + emoji) inside a dir=auto span for RTL.
- Auto-translate flips foreign messages whose model is already
  downloaded; first-time downloads keep the manual chip (user gesture).

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

src/app/components/message/content/FallbackContent.tsx

@@ -89,3 +89,47 @@ export const MessageEditedContent = as<
     </Text>
   ),
 );
+
+type TranslatedStatus = 'downloading' | 'translating' | 'done' | 'error';
+
+const translatedLabel = (
+  status: TranslatedStatus,
+  fromLangName: string,
+  downloadProgress?: number,
+): string => {
+  if (status === 'downloading') {
+    const pct =
+      typeof downloadProgress === 'number' ? ` ${Math.round(downloadProgress * 100)}%` : '';
+    return `Downloading translation model…${pct}`;
+  }
+  if (status === 'translating') return 'Translating…';
+  if (status === 'error') return 'Translation failed — show original';
+  return `Translated from ${fromLangName} · Show original`;
+};
+
+// Inline chip shown beside a translated message body. Clicking it toggles back
+// to the original text (the same per-event toggle the message menu drives).
+export const MessageTranslatedContent = as<
+  'span',
+  {
+    children?: never;
+    fromLangName: string;
+    status: TranslatedStatus;
+    downloadProgress?: number;
+    onToggle: () => void;
+  }
+>(({ fromLangName, status, downloadProgress, onToggle, ...props }, ref) => (
+  <span ref={ref} {...(props as React.HTMLAttributes<HTMLSpanElement>)}>
+    <button
+      type="button"
+      onClick={onToggle}
+      aria-pressed={status === 'done'}
+      aria-label="Toggle message translation"
+      style={{ cursor: 'pointer', background: 'none', border: 'none', padding: 0 }}
+    >
+      <Text as="span" size="T200" priority="300">
+        {` · ${translatedLabel(status, fromLangName, downloadProgress)}`}
+      </Text>
+    </button>
+  </span>
+));