🚀 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

eslint.config.mjs

@@ -9,8 +9,8 @@ const compat = new FlatCompat({
   baseDirectory: __dirname,
 });
 
-const eslintConfig = [
-  ...compat.extends("next/core-web-vitals", "next/typescript"),
-];
+const eslintConfig = [{
+  ignores: ["node_modules/**", ".next/**", "out/**", "build/**", "next-env.d.ts"]
+}, ...compat.extends("next/core-web-vitals", "next/typescript")];
 
 export default eslintConfig;