feat: add platform selection frontend for platform admins

Frontend implementation of platform admin flow:
- Update login.js to check for platform selection after login
- Add platform selection page (/admin/select-platform)
- Add platform context indicator in admin header
- Add is_super_admin to UserResponse schema
- Show "Super Admin" badge or platform name with switch option

Platform admins now:
1. Login normally at /admin/login
2. Get redirected to /admin/select-platform if they have multiple platforms
3. See current platform in header with option to switch

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/schema/auth.py

@@ -25,6 +25,7 @@ class UserResponse(BaseModel):
     username: str
     role: str
     is_active: bool
+    is_super_admin: bool = False
     preferred_language: str | None = None
     last_login: datetime | None = None
     created_at: datetime