Problem: language preference stored only in localStorage, lost on
'clear site data'. French users with browser lang=fr always got French
UI even when profile was English.
Solution:
- Backend: add 'language' field to user profile (default 'fr')
- Backend: PATCH /api/auth/me to update language preference
- Frontend: initI18n() priority: server > localStorage > navigator > 'fr'
- Frontend: initAuth() runs before initI18n() so server check works
- Frontend: profile Save persists language to server via PATCH
Verified: tabs show English after server profile set to 'en'