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/migrations/20260411125349_add_banned_to_user/migration.sql

@@ -0,0 +1,17 @@
+-- RedefineTables
+PRAGMA defer_foreign_keys=ON;
+PRAGMA foreign_keys=OFF;
+CREATE TABLE "new_User" (
+    "id" TEXT NOT NULL PRIMARY KEY,
+    "name" TEXT NOT NULL,
+    "email" TEXT NOT NULL,
+    "password" TEXT NOT NULL,
+    "banned" BOOLEAN NOT NULL DEFAULT false,
+    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+INSERT INTO "new_User" ("createdAt", "email", "id", "name", "password") SELECT "createdAt", "email", "id", "name", "password" FROM "User";
+DROP TABLE "User";
+ALTER TABLE "new_User" RENAME TO "User";
+CREATE UNIQUE INDEX "User_email_key" ON "User"("email");
+PRAGMA foreign_keys=ON;
+PRAGMA defer_foreign_keys=OFF;