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>

backend/src/services/buchhaltung.service.ts

@@ -653,6 +653,36 @@ async function createKonto(
   }
 }
 
+async function cascadeBudgetTypToChildren(parentId: number, newType: string): Promise<void> {
+  const children = await pool.query(
+    'SELECT id, budget_typ, budget_gwg, budget_anlagen, budget_instandhaltung, budget_gesamt FROM buchhaltung_konten WHERE parent_id = $1',
+    [parentId]
+  );
+
+  for (const child of children.rows) {
+    const oldType = child.budget_typ || 'detailliert';
+    if (oldType === newType) continue;
+
+    let gwg = 0, anlagen = 0, instandhaltung = 0, gesamt = 0;
+
+    if (oldType === 'detailliert' && newType === 'einfach') {
+      // Sum detail fields into gesamt
+      gesamt = (parseFloat(child.budget_gwg) || 0) + (parseFloat(child.budget_anlagen) || 0) + (parseFloat(child.budget_instandhaltung) || 0);
+    }
+    // einfach → detailliert: zero everything (user redistributes manually)
+
+    await pool.query(
+      `UPDATE buchhaltung_konten
+       SET budget_typ = $1, budget_gwg = $2, budget_anlagen = $3, budget_instandhaltung = $4, budget_gesamt = $5
+       WHERE id = $6`,
+      [newType, gwg, anlagen, instandhaltung, gesamt, child.id]
+    );
+
+    // Recurse into grandchildren
+    await cascadeBudgetTypToChildren(child.id, newType);
+  }
+}
+
 async function updateKonto(
   id: number,
   data: { konto_typ_id?: number; kontonummer?: string; bezeichnung?: string; parent_id?: number | null; budget_gwg?: number; budget_anlagen?: number; budget_instandhaltung?: number; notizen?: string; kategorie_id?: number | null; budget_typ?: string; budget_gesamt?: number }
@@ -711,6 +741,15 @@ async function updateKonto(
       `UPDATE buchhaltung_konten SET ${fields.join(', ')} WHERE id = $${idx} RETURNING *`,
       values
     );
+
+    // Cascade budget_typ change to all children recursively
+    if (data.budget_typ !== undefined && result.rows[0]) {
+      const newBudgetTyp = result.rows[0].budget_typ;
+      // Fetch what the old type was before the update (the RETURNING row has the new value)
+      // We know a change happened if the user sent budget_typ, so cascade unconditionally
+      await cascadeBudgetTypToChildren(id, newBudgetTyp);
+    }
+
     return result.rows[0] || null;
   } catch (error: any) {
     logger.error('BuchhaltungService.updateKonto failed', { error, id });