Backend:
- Add customerNumber field to AppUser with unique index
- Add customer number generation starting at K1000
- Add InvoiceService for creating invoices with PDF data
- Add InvoiceController with endpoints for listing customers and generating invoice data
- Update CatalogService to assign customer numbers to new users
- Update SampleService to preserve customerNumber on updates
- Fix SecurityConfig to enable method security and admin role checks
Frontend:
- Update UserOption/UserRow types to include customerNumber
- Add customer selection dialog in InvoiceManagementPage
- Add PDF generation and preview for invoices
- Fix encodePdfText function for proper PDF encoding
- Fix LocalStorage key for auth token