i18n Phase 4 — mobile (react-i18next)

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 5 (extension).

Scope (~3–5h):

• Install react-i18next, i18next, expo-localization.
• Resolver: profiles.locale (authed) → expo-secure-store override → expo-localization.getLocales()[0] → 'en'.
• Settings screen language picker writes both DB (via /api/me/locale) and secure-store (offline-first).
• Server-side push notifications read recipient profiles.locale when rendering body.
• Phase-6 string extraction FOR MOBILE belongs here.