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/components/common/PageBreadcrumbs.tsx

@@ -0,0 +1,44 @@
+import { Breadcrumbs, Typography, Link as MuiLink } from '@mui/material';
+import NavigateNextIcon from '@mui/icons-material/NavigateNext';
+import { Link } from 'react-router-dom';
+
+export interface BreadcrumbItem {
+  label: string;
+  href?: string;
+}
+
+interface Props {
+  items: BreadcrumbItem[];
+}
+
+export const PageBreadcrumbs = ({ items }: Props) => {
+  return (
+    <Breadcrumbs
+      separator={<NavigateNextIcon fontSize="small" />}
+      sx={{ mb: 2, fontSize: '0.8125rem' }}
+    >
+      {items.map((item, index) => {
+        const isLast = index === items.length - 1;
+        if (isLast || !item.href) {
+          return (
+            <Typography key={index} variant="body2" color="text.primary" fontWeight={500}>
+              {item.label}
+            </Typography>
+          );
+        }
+        return (
+          <MuiLink
+            key={index}
+            component={Link}
+            to={item.href}
+            underline="hover"
+            color="text.secondary"
+            variant="body2"
+          >
+            {item.label}
+          </MuiLink>
+        );
+      })}
+    </Breadcrumbs>
+  );
+};

frontend/src/components/common/index.ts

@@ -0,0 +1,2 @@
+export { PageBreadcrumbs } from './PageBreadcrumbs';
+export type { BreadcrumbItem } from './PageBreadcrumbs';