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/services/admin.ts

@@ -31,4 +31,5 @@ export const adminApi = {
   fdiskSyncLogs: () => api.get<ApiResponse<FdiskSyncLogsResponse>>('/api/admin/fdisk-sync/logs').then(r => r.data.data),
   fdiskSyncTrigger: (force = false) => api.post<ApiResponse<{ started: boolean }>>('/api/admin/fdisk-sync/trigger', { force }).then(r => r.data.data),
   purgeFdiskData: (userId: string) => api.delete<ApiResponse<{ profileFieldsCleared: number; ausbildungen: number; befoerderungen: number; untersuchungen: number; fahrgenehmigungen: number }>>(`/api/admin/users/${userId}/fdisk-data`).then(r => r.data.data),
+  purgeUserData: (userId: string) => api.post<ApiResponse<Record<string, number>>>(`/api/admin/users/${userId}/purge-data`).then(r => r.data.data),
 };

frontend/src/services/auth.ts

@@ -8,6 +8,7 @@ export interface AuthCallbackResponse {
   token: string;
   refreshToken: string;
   user: User;
+  isNewUser?: boolean;
 }
 
 // The backend returns camelCase field names; map them to the snake_case User type.
@@ -45,7 +46,7 @@ export const authService = {
     const response = await api.post<{
       success: boolean;
       message: string;
-      data: { accessToken: string; refreshToken: string; user: Record<string, unknown> };
+      data: { accessToken: string; refreshToken: string; isNewUser?: boolean; user: Record<string, unknown> };
     }>('/api/auth/callback', {
       code,
       redirect_uri: REDIRECT_URI,
@@ -54,6 +55,7 @@ export const authService = {
       token: response.data.data.accessToken,
       refreshToken: response.data.data.refreshToken,
       user: mapBackendUser(response.data.data.user),
+      isNewUser: response.data.data.isNewUser,
     };
   },