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/BuchhaltungKontoDetail.tsx

@@ -9,6 +9,7 @@ import {
 } from '@mui/material';
 import { ArrowBack, KeyboardArrowDown, KeyboardArrowUp } from '@mui/icons-material';
 import DashboardLayout from '../components/dashboard/DashboardLayout';
+import { PageBreadcrumbs } from '../components/common';
 import { buchhaltungApi } from '../services/buchhaltung';
 import { AUSGABEN_TYP_LABELS } from '../types/buchhaltung.types';
 import type { AusgabenTyp, BuchhaltungAudit } from '../types/buchhaltung.types';
@@ -132,6 +133,10 @@ export default function BuchhaltungKontoDetail() {
   return (
     <DashboardLayout>
       <Box sx={{ p: 3 }}>
+        <PageBreadcrumbs items={[
+          { label: 'Buchhaltung', href: '/buchhaltung' },
+          { label: `${konto.kontonummer} — ${konto.bezeichnung}` },
+        ]} />
         <Box sx={{ display: 'flex', alignItems: 'center', gap: 1, mb: 3 }}>
           <Button startIcon={<ArrowBack />} onClick={() => navigate('/buchhaltung')}>
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