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/main.py

@@ -9,7 +9,7 @@ from app.config import get_settings
 from app.database import engine
 from app.models import Base
 from app.routers import auth, users, children, subjects, schedules, sessions, logs, dashboard
-from app.routers import morning_routine, break_activity
+from app.routers import morning_routine, break_activity, admin
 from app.websocket.manager import manager
 
 settings = get_settings()
@@ -66,6 +66,7 @@ app.include_router(logs.router)
 app.include_router(morning_routine.router)
 app.include_router(break_activity.router)
 app.include_router(dashboard.router)
+app.include_router(admin.router)
 
 
 @app.get("/api/health")