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/app/(authenticated)/settings/sessions/page.test.tsx

@@ -3,7 +3,7 @@
  * Smoke tests for page rendering
  */
 
-import { render, screen, waitFor } from '@testing-library/react';
+import { render, screen } from '@testing-library/react';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import SessionsPage from '@/app/(authenticated)/settings/sessions/page';