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>

docker-compose.yml

@@ -29,6 +29,8 @@ services:
       ACCESS_TOKEN_EXPIRE_MINUTES: ${ACCESS_TOKEN_EXPIRE_MINUTES:-30}
       REFRESH_TOKEN_EXPIRE_DAYS: ${REFRESH_TOKEN_EXPIRE_DAYS:-30}
       CORS_ORIGINS: ${CORS_ORIGINS:-http://localhost:8057}
+      ADMIN_USERNAME: ${ADMIN_USERNAME:-admin}
+      ADMIN_PASSWORD: ${ADMIN_PASSWORD:-change_me_admin_password}
     depends_on:
       db:
         condition: service_healthy