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/types/auth.types.ts

@@ -21,8 +21,12 @@ export interface AuthState {
   isLoading: boolean;
 }
 
+export interface LoginResult {
+  isNewUser?: boolean;
+}
+
 export interface AuthContextType extends AuthState {
-  login: (code: string) => Promise<void>;
+  login: (code: string) => Promise<LoginResult>;
   logout: () => void;
   refreshAuth: () => Promise<void>;
 }