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/errors.test.ts

@@ -630,7 +630,7 @@ describe('API Error Handling', () => {
 
   describe('Error message completeness', () => {
     it('should have non-empty messages for all error codes', () => {
-      Object.entries(ERROR_MESSAGES).forEach(([code, message]) => {
+      Object.entries(ERROR_MESSAGES).forEach(([_code, message]) => {
         expect(message).toBeTruthy();
         expect(message.length).toBeGreaterThan(0);
         expect(message).not.toBe('');
@@ -639,7 +639,7 @@ describe('API Error Handling', () => {
 
     it('should have user-friendly messages', () => {
       // Check that messages don't contain technical jargon or error codes
-      Object.entries(ERROR_MESSAGES).forEach(([code, message]) => {
+      Object.entries(ERROR_MESSAGES).forEach(([_code, message]) => {
         expect(message).not.toContain('null');
         expect(message).not.toContain('undefined');
         expect(message).not.toContain('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,