feat: server-side language preference — survives clear site data

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'

backend/auth/user_store.py

@@ -88,6 +88,7 @@ def create_user(
         "role": role,
         "vaults": vaults or [],
         "active": True,
+        "language": "fr",  # default UI language
         "created_at": datetime.now(timezone.utc).isoformat(),
         "last_login": None,
         "failed_attempts": 0,