Refactor ESLint configuration and update test rules for clarity and consistency

- Consolidated and modularized `eslint.config.mjs` with defined rules for source, test, E2E, and scripts.
- Improved test and E2E rules with relaxed settings for flexibility and enhanced mocking.
- Standardized variable naming and removed redundant imports in unit and E2E tests.
- Updated error handling and comments to align with modern TypeScript best practices (e.g., `@ts-expect-error`).

frontend/tests/components/layout/Skeletons.test.tsx

@@ -30,7 +30,7 @@ describe('HeaderSkeleton', () => {
   });
 
   it('has proper styling classes', () => {
-    const { container } = render(<HeaderSkeleton />);
+    render(<HeaderSkeleton />);
 
     // Verify backdrop blur and background
     const header = screen.getByRole('banner');
@@ -60,7 +60,7 @@ describe('AuthLoadingSkeleton', () => {
   });
 
   it('renders main content with container', () => {
-    const { container } = render(<AuthLoadingSkeleton />);
+    render(<AuthLoadingSkeleton />);
 
     const main = screen.getByRole('main');
     expect(main).toHaveClass('flex-1');
@@ -71,7 +71,7 @@ describe('AuthLoadingSkeleton', () => {
   });
 
   it('renders skeleton placeholders in main content', () => {
-    const { container } = render(<AuthLoadingSkeleton />);
+    render(<AuthLoadingSkeleton />);
 
     const main = screen.getByRole('main');