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/contexts/AuthContext.tsx

@@ -1,5 +1,5 @@
 import React, { createContext, useCallback, useContext, useState, useEffect, ReactNode } from 'react';
-import { AuthContextType, AuthState } from '../types/auth.types';
+import { AuthContextType, AuthState, LoginResult } from '../types/auth.types';
 import { authService } from '../services/auth';
 import { getToken, setToken, removeToken, getUser, setUser, removeUser, setRefreshToken, removeRefreshToken } from '../utils/storage';
 import { useNotification } from './NotificationContext';
@@ -67,11 +67,11 @@ export const AuthProvider: React.FC<AuthProviderProps> = ({ children }) => {
     initializeAuth();
   }, []);
 
-  const login = useCallback(async (code: string): Promise<void> => {
+  const login = useCallback(async (code: string): Promise<LoginResult> => {
     try {
       setState((prev) => ({ ...prev, isLoading: true }));
 
-      const { token, refreshToken, user } = await authService.handleCallback(code);
+      const { token, refreshToken, user, isNewUser } = await authService.handleCallback(code);
 
       // Save to localStorage
       setToken(token);
@@ -88,6 +88,8 @@ export const AuthProvider: React.FC<AuthProviderProps> = ({ children }) => {
 
       // Show success notification
       notification.showSuccess('Anmeldung erfolgreich');
+
+      return { isNewUser };
     } catch (error) {
       console.error('Login failed:', error);
       setState({