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/components/layout/Sidebar.tsx

@@ -87,9 +87,9 @@ export function Sidebar() {
       permission: item.to === '/dashboard' ? 'dashboard:read' : item.to === '/contacts' ? 'contacts:read' : undefined,
     }));
 
-    const pluginItems = manifests
-      .flatMap((m) => m.menu_items)
-      .filter(item => !item.permission || canAccess(item.permission))
+    const pluginItems = (manifests || [])
+      .flatMap((m) => (Array.isArray(m.menu_items) ? m.menu_items : []))
+      .filter((item): item is NonNullable<typeof item> => !!item && (!item.permission || canAccess(item.permission)))
       .map(item => ({
         path: item.path,
         labelKey: item.label_key,