Refactor Playwright tests to use cached authentication state for improved performance

- Removed redundant `loginViaUI` calls across E2E tests, leveraging cached storage state for faster test execution.
- Enhanced Playwright configuration to include a `setup` project for pre-caching admin and regular user authentication states.
- Added new `auth.setup.ts` to handle initial authentication and save storage states to `.auth` directory.
- Increased local worker count to 16 (CI unchanged) to optimize parallel execution.
- Updated `.gitignore` to exclude authentication state files.

frontend/e2e/settings-password.spec.ts

@@ -12,7 +12,7 @@ test.describe('Password Change', () => {
     await setupAuthenticatedMocks(page);
 
     // Login via UI to establish authenticated session
-    await loginViaUI(page);
+    // Auth already cached in storage state (loginViaUI removed for performance)
 
     // Navigate to password page
     await page.goto('/settings/password', { waitUntil: 'networkidle' });