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/mail/MailGroupPanel.tsx

@@ -73,7 +73,7 @@ export interface GroupedMails {
 }
 
 export function applyGrouping(mails: any[], groupState: GroupState): GroupedMails[] {
-  if (!groupState.conditions.length) return [{ key: 'all', label: 'Alle', mails }];
+  if (!(groupState.conditions?.length ?? 0)) return [{ key: 'all', label: 'Alle', mails }];
   const defs = GROUP_FIELDS;
 
   function groupRecursive(items: any[], conditions: GroupCondition[], depth: number): GroupedMails[] {
@@ -139,7 +139,7 @@ export function MailGroupPanel({ groupState, onGroupChange }: MailGroupPanelProp
     setOpen(!open);
   };
 
-  const activeCount = groupState.conditions.length;
+  const activeCount = groupState.conditions?.length ?? 0;
 
   const addCondition = () => {
     onGroupChange({ conditions: [...groupState.conditions, { id: newGroupId(), field: 'from' }] });