feat(tests): add Playwright browser extension e2e tests
- Add tests/fixtures/extension-app.ts with Chrome/Firefox support
- launchExtension() launches browser with extension loaded
- getSharedExtension()/closeSharedExtension() for shared instance
- Chrome uses --load-extension flag, Firefox uses temp profile
- Auto-detects extension ID from service worker URL
- Add tests/helpers/extension-utils.ts with popup/options helpers
- waitForCommandInput(), typeCommand(), tabComplete(), executeCommand()
- waitForOptionsInit(), fillSyncConfig(), getDiagnostics()
- Toggle helpers for bookmark/tab/history sync features
- Add tests/extension/popup.spec.ts with 15 command bar tests
- Placeholder text, autocomplete suggestions
- Tab completion, Enter execution, Escape close
- Case preservation, args mode behavior
- Add tests/extension/options.spec.ts with 26 options page tests
- Page structure, sync config, diagnostics display
- Refresh button, feature toggles
- Update playwright.config.ts with extension-chrome/firefox projects
- Add package.json scripts:
- test:extension:browser (Chrome)
- test:extension:browser:firefox (Firefox)
- test:extension:browser:visible (headed mode)