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/components/theme/ThemeToggle.test.tsx

@@ -6,7 +6,7 @@
 import { render, screen, waitFor } from '@testing-library/react';
 import userEvent from '@testing-library/user-event';
 import { ThemeToggle } from '@/components/theme/ThemeToggle';
-import { ThemeProvider, useTheme } from '@/components/theme/ThemeProvider';
+import { useTheme } from '@/components/theme/ThemeProvider';
 
 // Mock theme provider for controlled testing
 jest.mock('@/components/theme/ThemeProvider', () => {