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/lib/api/hooks/useAuth.test.tsx

@@ -4,7 +4,7 @@
  * These tests cover hook setup, types, and basic integration
  */
 
-import { renderHook, waitFor } from '@testing-library/react';
+import { renderHook } from '@testing-library/react';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import {
   useIsAuthenticated,