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/src/pages/Settings.tsx

@@ -8,8 +8,9 @@ export function SettingsPage() {
   const { t } = useTranslation();
   const manifests = usePluginStore(s => s.manifests);
   const pluginSettingsPages = useMemo(
-    () => manifests
-      .flatMap((m) => m.settings_pages)
+    () => (manifests || [])
+      .flatMap((m) => (Array.isArray(m.settings_pages) ? m.settings_pages : []))
+      .filter((p): p is NonNullable<typeof p> => !!p && !!p.path)
       .sort((a, b) => a.order - b.order),
     [manifests]
   );