i18n Phase 5 — extension (react-i18next, bundled)

Split from the master i18n plan (task i18n-en, Plane YOPSYS-62).

BLOCKED ON: Phase 1 foundation (i18n-en) + Phase 2 DB/preference. Promote backlog → todo once BOTH merged. Parallel with Phase 3 (web) and Phase 4 (mobile).

Scope (~2–3h):

• Install react-i18next + i18next.
• Bundle BOTH locales statically — CSP forbids dynamic JSON fetch from chrome-extension://.
• Resolver: profiles.locale (after auth) → stored override (chrome.storage.local) → chrome.i18n.getUILanguage() → 'en'.
• Settings UI in sidepanel mirrors the mobile picker.
• Phase-6 string extraction FOR EXTENSION belongs here.