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/IssueDetail.tsx

@@ -264,6 +264,10 @@ export default function IssueDetail() {
         <PageHeader
           title={`${formatIssueId(issue)} — ${issue.titel}`}
           backTo="/issues"
+          breadcrumbs={[
+            { label: 'Issues', href: '/issues' },
+            { label: `${formatIssueId(issue)} — ${issue.titel}` },
+          ]}
           actions={
             <Chip
               label={getStatusLabel(statuses, issue.status)}