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/hooks/usePrefersReducedMotion.test.ts

@@ -164,7 +164,7 @@ describe('usePrefersReducedMotion', () => {
   it('handles SSR environment safely', () => {
     const originalWindow = global.window;
 
-    // @ts-ignore - Simulating SSR
+    // @ts-expect-error - Simulating SSR
     delete global.window;
 
     const { result } = renderHook(() => usePrefersReducedMotion());