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.

.gitignore

@@ -27,6 +27,10 @@ coverage
 # nyc test coverage
 .nyc_output
 
+# Playwright authentication state (contains test auth tokens)
+frontend/e2e/.auth/
+**/playwright/.auth/
+
 # Grunt intermediate storage (https://gruntjs.com/creating-plugins#storing-task-files)
 .grunt