feat(admin): add admin panel with user management, daily puzzle CRUD, and activity graph

- Add admin page at /admin (restricted to ADMIN_EMAIL via proxy)
- Implement user ban/unban and deletion
- Add daily puzzle creation, modification, deletion
- Include 14-day activity chart with hover details
- Add User.banned field to schema and migrate database
- Block banned users from logging in
- Add ADMIN_EMAIL to .env.local configuration
- Update Prisma client after schema changes

lib/generated/prisma/internal/prismaNamespaceBrowser.ts

@@ -75,6 +75,7 @@ export const UserScalarFieldEnum = {
   name: 'name',
   email: 'email',
   password: 'password',
+  banned: 'banned',
   createdAt: 'createdAt'
 } as const