test: Fix E2E Playwright tests (25/34 pass) + cleanup old briefings

E2E Test Fixes (12 tests fixed, 25/34 now pass):
- helpers.ts: Fix API mock routes, response shapes, welcome dialog dismissal
- auth.spec.ts: Fix logout selector (duplicate button match)
- calendar.spec.ts: Fix strict mode violations, modal close assertions
- dms.spec.ts: Fix strict mode violations, modal close assertions
- contact-crud.spec.ts: Fix modal close assertion
- mail.spec.ts: Fix modal close assertion
- search.spec.ts: Fix search result expectations, empty query test

9 remaining failures: Playwright route interception with glob patterns
does not match when Vite dev proxy is configured (calendar/mail/plugins).

Cleanup:
- Delete 13 old .a0/briefings/ files
- Delete test-results/, docs/test_raw_output.md, e2e_test_report.md, test_report.md
- Delete .a0/known_errors.md (circuit breaker bug is fixed)

frontend/src/main.tsx

@@ -13,6 +13,12 @@ if ('serviceWorker' in navigator) {
   });
 }
 
+// Expose auth store globally in dev mode for E2E test access
+import { useAuthStore } from './store/authStore';
+if (import.meta.env.DEV) {
+  (window as any).__AUTH_STORE__ = useAuthStore;
+}
+
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
     <App />