- Add 'Rechnung' menu with sub-items 'Verwalten' and 'Template' in admin sidebar
- Create InvoiceTemplatePage with drag-and-drop editor for invoice templates
- Includes invoice-specific elements (header, customer data, issuer info,
invoice items, totals, payment terms, bank details)
- Supports PDF preview and download
- API integration for saving/loading templates (/admin/invoice-template)
- Create InvoiceManagementPage as placeholder for invoice overview
- Add routes for /admin/rechnung/verwalten and /admin/rechnung/template
- Update page titles in AppShell for new routes
