fix: prevent config creation hang on pricelist sync

SyncPricelistsIfNeeded was called synchronously in Create(), blocking
the HTTP response for several seconds while pricelist data was fetched.
Users clicking multiple times caused 6+ duplicate configurations.

- Run SyncPricelistsIfNeeded in a goroutine so Create() returns immediately
- Add TryLock mutex to SyncPricelistsIfNeeded to skip concurrent calls

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/services/local_configuration.go

@@ -49,11 +49,13 @@ func NewLocalConfigurationService(
 
 // Create creates a new configuration in local SQLite and queues it for sync
 func (s *LocalConfigurationService) Create(ownerUsername string, req *CreateConfigRequest) (*models.Configuration, error) {
-	// If online, check for new pricelists first
+	// If online, trigger pricelist sync in the background — do not block config creation
 	if s.isOnline() {
-		if err := s.syncService.SyncPricelistsIfNeeded(); err != nil {
-			// Log but don't fail - we can still use local pricelists
-		}
+		go func() {
+			if err := s.syncService.SyncPricelistsIfNeeded(); err != nil {
+				// Log but don't fail - we can still use local pricelists
+			}
+		}()
 	}
 
 	projectUUID, err := s.resolveProjectUUID(ownerUsername, req.ProjectUUID)