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/(auth)/password-reset/page.test.tsx

@@ -9,7 +9,7 @@ import PasswordResetPage from '@/app/(auth)/password-reset/page';
 // Mock dynamic import
 jest.mock('next/dynamic', () => ({
   __esModule: true,
-  default: (importFn: () => Promise<any>, options?: any) => {
+  default: (_importFn: () => Promise<any>, _options?: any) => {
     const Component = () => <div data-testid="password-reset-form">Mocked PasswordResetRequestForm</div>;
     Component.displayName = 'PasswordResetRequestForm';
     return Component;