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

auth.ts

@@ -21,6 +21,7 @@ export const { handlers, auth, signIn, signOut } = NextAuth({
           user.password,
         );
         if (!valid) return null;
+        if (user.banned) return null;
         return { id: user.id, name: user.name, email: user.email };
       },
     }),
@@ -31,11 +32,17 @@ export const { handlers, auth, signIn, signOut } = NextAuth({
   },
   callbacks: {
     jwt({ token, user }) {
-      if (user) token.id = user.id;
+      if (user) {
+        token.id = user.id;
+        token.email = user.email;
+      }
       return token;
     },
     session({ session, token }) {
-      if (session.user) session.user.id = token.id as string;
+      if (session.user) {
+        session.user.id = token.id as string;
+        session.user.email = token.email as string;
+      }
       return session;
     },
   },