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/BestellungDetail.tsx

@@ -673,6 +673,10 @@ export default function BestellungDetail() {
       <PageHeader
         title={bestellung.bezeichnung}
         backTo="/bestellungen"
+        breadcrumbs={[
+          { label: 'Bestellungen', href: '/bestellungen' },
+          { label: bestellung.bezeichnung },
+        ]}
         actions={
           <Box sx={{ display: 'flex', alignItems: 'center', gap: 1 }}>
             {canExport && !editMode && (