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/state/settings.ts

@@ -267,6 +267,10 @@ export interface Settings {
     | 'earthday'
     | 'deepspace'
     | 'arcade';
+
+  // On-device message translation
+  translateTargetLang: string; // BCP-47 base code, default 'en'
+  autoTranslate: boolean; // auto-translate incoming foreign messages (opt-in)
 }
 
 const defaultSettings: Settings = {
@@ -366,6 +370,9 @@ const defaultSettings: Settings = {
   soundboardVolume: 80,
 
   seasonalThemeOverride: 'auto',
+
+  translateTargetLang: 'en',
+  autoTranslate: false,
 };
 
 export const getSettings = (): Settings => {
@@ -402,6 +409,16 @@ export const getSettings = (): Settings => {
         saved.ringtoneId === 'none'
           ? saved.ringtoneId
           : defaultSettings.ringtoneId,
+      // Coerce persisted target language to a non-empty string; anything else
+      // (missing/wrong type) falls back to the default.
+      translateTargetLang:
+        typeof saved.translateTargetLang === 'string' && saved.translateTargetLang.trim()
+          ? saved.translateTargetLang
+          : defaultSettings.translateTargetLang,
+      autoTranslate:
+        typeof saved.autoTranslate === 'boolean'
+          ? saved.autoTranslate
+          : defaultSettings.autoTranslate,
       composerToolbarButtons: {
         ...DEFAULT_COMPOSER_TOOLBAR,
         ...(saved.composerToolbarButtons ?? {}),