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/router.py

@@ -242,10 +242,41 @@ async def get_me(current_user=Depends(require_auth)):
         "display_name": current_user["display_name"],
         "role": current_user["role"],
         "vaults": current_user["vaults"],
+        "language": current_user.get("language", "fr"),
         "last_login": current_user.get("last_login"),
     }
 
 
+class UpdateMeRequest(BaseModel):
+    """Fields the user can update on their own profile."""
+    display_name: Optional[str] = None
+    language: Optional[str] = None
+
+
+@router.patch("/me")
+async def patch_me(req: UpdateMeRequest, current_user=Depends(require_auth)):
+    """Update current user's profile fields (display_name, language)."""
+    from .user_store import update_user
+    updates = {}
+    if req.display_name is not None:
+        updates["display_name"] = req.display_name
+    if req.language is not None:
+        if req.language not in ("fr", "en"):
+            raise HTTPException(400, "language must be 'fr' or 'en'")
+        updates["language"] = req.language
+    if not updates:
+        raise HTTPException(400, "No fields to update")
+    updated = update_user(current_user["username"], updates)
+    return {
+        "username": updated["username"],
+        "display_name": updated["display_name"],
+        "role": updated["role"],
+        "vaults": updated["vaults"],
+        "language": updated.get("language", "fr"),
+        "last_login": updated.get("last_login"),
+    }
+
+
 @router.post("/change-password")
 async def change_password(
     req: ChangePasswordRequest,

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,