fix(frontend): E2E-Test-Suite vollständig grün machen

- Robuster gegen undefined API-Daten in Mail, ContactDetail, ContactsList, Settings, Sidebar
- E2E-Mocks korrigiert für Kontakt-Detail, Mail-Liste/Folders und Plugin-Toggle
- Auth-Store mit persist-Middleware für E2E-Login
- test-results/ in .gitignore aufgenommen

Playwright E2E: 34/34 passed

frontend/e2e/search.spec.ts

@@ -25,7 +25,7 @@ test.describe('Global Search E2E', () => {
     await page.locator('[data-testid="search-submit-btn"]').click();
 
     // Wait for results
-    await expect(page.locator('[data-testid="search-results-list"], [data-testid="search-results-all"]')).toBeVisible({ timeout: 10_000 });
+    await expect(page.locator('[data-testid="search-results-list"], [data-testid="search-results-all"]').first()).toBeVisible({ timeout: 10_000 });
 
     // Should show at least one result (or empty state if mock doesn't match exactly)
     const resultItem = page.locator('[data-testid^="search-result-"]').first();