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/e2e/homepage.spec.ts

@@ -214,7 +214,7 @@ test.describe('Homepage - Mobile Menu Interactions', () => {
 
   test.skip('should close mobile menu when clicking outside', async ({ page }) => {
     // Open mobile menu
-    const mobileMenu = await openMobileMenu(page);
+    const _mobileMenu = await openMobileMenu(page);
 
     // Press Escape key to close menu (more reliable than clicking overlay)
     await page.keyboard.press('Escape');