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

prisma/schema.prisma

@@ -12,6 +12,7 @@ model User {
   name         String
   email        String        @unique
   password     String
+  banned       Boolean       @default(false)
   createdAt    DateTime      @default(now())
   games        Game[]
   dailyResults DailyResult[]