🚀 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

jest.setup.ts

@@ -9,20 +9,26 @@ jest.mock("react-responsive-masonry", () => ({
   default: ({ children }: { children: React.ReactNode }) =>
     React.createElement("div", null, children),
   get ResponsiveMasonry() {
-    return ({ children }: { children: React.ReactNode }) =>
+    const ResponsiveMasonryComponent = ({ children }: { children: React.ReactNode }) =>
       React.createElement("div", null, children);
+    ResponsiveMasonryComponent.displayName = 'ResponsiveMasonry';
+    return ResponsiveMasonryComponent;
   },
 }));
 
 // Mock next/link
 jest.mock('next/link', () => {
-  return ({ children }: { children: React.ReactNode }) => children;
+  const LinkComponent = ({ children }: { children: React.ReactNode }) => children;
+  LinkComponent.displayName = 'Link';
+  return LinkComponent;
 });
 
 // Mock next/image
 jest.mock('next/image', () => {
-  return ({ src, alt, ...props }: any) => 
+  const ImageComponent = ({ src, alt, ...props }: Record<string, unknown>) => 
     React.createElement('img', { src, alt, ...props });
+  ImageComponent.displayName = 'Image';
+  return ImageComponent;
 });
 
 // Custom render function with ToastProvider