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/HaushaltsplanDetail.tsx

@@ -1,5 +1,6 @@
 import { useState } from 'react';
 import DashboardLayout from '../components/dashboard/DashboardLayout';
+import { PageBreadcrumbs } from '../components/common';
 import {
   Alert,
   Box,
@@ -143,6 +144,10 @@ export default function HaushaltsplanDetail() {
 
   return (
     <DashboardLayout>
+      <PageBreadcrumbs items={[
+        { label: 'Haushaltsplan', href: '/haushaltsplan' },
+        { label: planung.bezeichnung },
+      ]} />
       <Box sx={{ display: 'flex', alignItems: 'center', gap: 1, mb: 3 }}>
         <IconButton onClick={() => navigate('/haushaltsplan')}><ArrowBack /></IconButton>
         <Typography variant="h4" sx={{ flexGrow: 1 }}>{planung.bezeichnung}</Typography>