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/confirm/PasswordResetConfirmContent.test.tsx

@@ -25,7 +25,7 @@ jest.mock('next/link', () => ({
 // Mock dynamic import
 jest.mock('next/dynamic', () => ({
   __esModule: true,
-  default: (importFn: () => Promise<any>, options?: any) => {
+  default: (_importFn: () => Promise<any>, _options?: any) => {
     const Component = ({ onSuccess }: { onSuccess?: () => void }) => (
       <div data-testid="password-reset-confirm-form">
         <button onClick={onSuccess}>Submit</button>