Add Super Admin panel with user impersonation

- New /super-admin/login and /super-admin routes with separate auth
- Super admin can view all registered accounts and impersonate any user
- Impersonation banner shows at top of screen with exit button
- ADMIN_USERNAME and ADMIN_PASSWORD config added to .env and docker-compose.yml
- Fixed auth store: export setToken, clearToken, and setUser so they are
  accessible from superAdmin store
- Updated README with super admin feature, new env vars, and setup notes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/config.py

@@ -9,6 +9,8 @@ class Settings(BaseSettings):
     access_token_expire_minutes: int = 30
     refresh_token_expire_days: int = 30
     cors_origins: str = "http://localhost:8054"
+    admin_username: str = "admin"
+    admin_password: str = "change_me_admin_password"
 
     @property
     def cors_origins_list(self) -> list[str]: