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>

frontend/src/stores/auth.js

@@ -64,11 +64,18 @@ export const useAuthStore = defineStore('auth', () => {
     }
   }
 
+  function setUser(userData) {
+    user.value = userData
+  }
+
   return {
     accessToken,
     user,
     isAuthenticated,
     timezone,
+    setToken,
+    clearToken,
+    setUser,
     login,
     register,
     logout,