Frontend:
- Extend UserManagementPage for both ADMIN and primary users
- Admins see all primary users (excluding other admins)
- Primary users see only their sub-users
- Add create sub-user form for primary users
- Adjust UI text based on user role
- Fix table columns (hide company column for primary users)
- Set active=false for newly registered users
- Return RegistrationResponse instead of SessionResponse after registration
- Show success message informing user that admin approval is pending
- Login check already filters for active users only
- Add nextSampleNumber field to AppUser record with default 100000
- Add compact constructor to initialize nextSampleNumber to 100000
- Add reserveNextSampleNumber method to atomically reserve and increment
- Update createSample to use reserveNextSampleNumber from user
- Update nextSampleNumber to read from user collection
- Update dashboardOverview to use new nextSampleNumber method
- Add findTopByOwnerAccountIdOrderBySampleNumberDesc to SampleRepository
- Modify nextSampleNumber to accept ownerAccountId parameter
- Calculate next sample number per user based on ownerAccountId
- Update dashboardOverview and createSample to use user-specific numbering
- 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
