feat: Add invoice items element with dynamic pricing from pricing table

- Add new 'invoice-items-muh' element to invoice template
- Add 'Positionen' category back to palette groups
- Create helper functions to format price and generate invoice content
- Add /current endpoint to SystemPricingController for authenticated users
- Load pricing when creating starter layout and display:
  - Monatliche Systemgebühr MUH with net price
  - Umsatzsteuer (19%)
  - Gesamtsumme (brutto)
- Update starter layout positioning for new element

backend/src/main/java/de/svencarstensen/muh/web/SystemPricingController.java

@@ -9,7 +9,6 @@ import java.util.Optional;
 
 @RestController
 @RequestMapping("/api/admin/pricing")
-@PreAuthorize("hasRole('ADMIN')")
 public class SystemPricingController {
 
     private final SystemPricingService systemPricingService;
@@ -19,13 +18,23 @@ public class SystemPricingController {
     }
 
     @GetMapping
+    @PreAuthorize("hasRole('ADMIN')")
     public PricingResponse getPricing() {
         Optional<SystemPricing> pricing = systemPricingService.getCurrentPricing();
         return pricing.map(p -> new PricingResponse(p.monthlyPrice(), p.updatedAt().toString()))
                 .orElseGet(() -> new PricingResponse(null, null));
     }
 
+    @GetMapping("/current")
+    @PreAuthorize("isAuthenticated()")
+    public PricingResponse getCurrentPricing() {
+        Optional<SystemPricing> pricing = systemPricingService.getCurrentPricing();
+        return pricing.map(p -> new PricingResponse(p.monthlyPrice(), p.updatedAt().toString()))
+                .orElseGet(() -> new PricingResponse(null, null));
+    }
+
     @PostMapping
+    @PreAuthorize("hasRole('ADMIN')")
     public PricingResponse savePricing(@RequestBody PricingRequest request) {
         SystemPricing saved = systemPricingService.savePricing(request.monthlyPrice());
         return new PricingResponse(saved.monthlyPrice(), saved.updatedAt().toString());