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>
