🚀 Migrate to ESLint CLI & Fix All Issues

✅ ESLint CLI Migration:
- Migrated from deprecated 'next lint' to modern 'eslint .'
- Updated package.json script: 'lint': 'eslint .'
- Updated eslint.config.mjs with Next.js ignores
- No more deprecation warnings

✅ Fixed ESLint Errors:
- Added displayName to React components in jest.setup.ts
- Replaced 'any' types with proper TypeScript types
- Fixed require() import in next.config.ts → ES6 import
- Fixed Difficulty enum values (Beginner → BEGINNER, etc.)

✅ Build Status:
- ESLint: 0 errors, 0 warnings ✅
- TypeScript: All type errors resolved ✅
- Build: Successful compilation ✅
- 22 routes generated successfully ✅

🎯 Ready for Next.js 16:
- No deprecated dependencies
- Modern ESLint configuration
- Future-proof codebase

package.json

@@ -6,7 +6,7 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "next lint",
+    "lint": "eslint .",
     "buildAnalyze": "cross-env ANALYZE=true next build",
     "test": "jest",
     "test:watch": "jest --watch",