feat: user data purge, breadcrumbs, first-login dialog, widget consolidation, bookkeeping cascade

- Admin can purge all personal data for a user (POST /api/admin/users/:userId/purge-data)
  while keeping the account; clears profile, notifications, bookings, ical tokens, preferences
- Add isNewUser flag to auth callback response; first-login dialog prompts for Standesbuchnummer
- Add PageBreadcrumbs component and apply to 18 sub-pages across the app
- Cascade budget_typ changes from parent pot to all children recursively, converting amounts
  (detailliert→einfach: sum into budget_gesamt; einfach→detailliert: zero all for redistribution)
- Migrate NextcloudTalkWidget to use shared WidgetCard template for consistent header styling

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/AdminSettings.tsx

@@ -32,6 +32,7 @@ import {
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { Navigate } from 'react-router-dom';
 import DashboardLayout from '../components/dashboard/DashboardLayout';
+import { PageBreadcrumbs } from '../components/common';
 import { usePermissionContext } from '../contexts/PermissionContext';
 import { useNotification } from '../contexts/NotificationContext';
 import { settingsApi } from '../services/settings';
@@ -293,6 +294,10 @@ function AdminSettings() {
   return (
     <DashboardLayout>
       <Container maxWidth="lg">
+        <PageBreadcrumbs items={[
+          { label: 'Admin', href: '/admin' },
+          { label: 'Einstellungen' },
+        ]} />
         <Typography variant="h4" gutterBottom sx={{ mb: 4 }}>
           Admin-Einstellungen
         </Typography>