Merge watch profiles into user accounts — one profile per login, no picker

- Removed the "Who's Watching" sub-profile picker entirely: ProfileProvider now
  auto-resolves the single profile tied to the account immediately after login
- Backend: removed POST/DELETE /profiles (no more creating/deleting extra
  profiles — lifecycle is entirely driven by user create/delete, already wired
  in the admin Users CRUD); kept GET (internal) and PATCH (edit your own
  profile's avatar/kids-mode/rating cap)
- Deleted ProfileSelect page and the now-unused ProtectedRoute component
- Navbar: removed the "switch profile" action, account avatar is now just a
  static display
- api.js: dropped the now-dead X-Profile-Id header logic (backend already
  falls back to the account's single profile when the header is absent)

backend/models.py

@@ -47,17 +47,10 @@ class AdminUserUpdate(BaseModel):
     password: Optional[str] = None
 
 
-# ---------- Profiles ("Who's Watching") ----------
+# ---------- Profile (1:1 with account) ----------
 RATING_ORDER = ["G", "PG", "PG-13", "R", "NR"]
 
 
-class ProfileCreate(BaseModel):
-    name: str
-    avatar_color: str = "#D9381E"
-    is_kids: bool = False
-    max_rating: str = "NR"  # one of RATING_ORDER
-
-
 class ProfileUpdate(BaseModel):
     name: Optional[str] = None
     avatar_color: Optional[str] = None