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/sync/service.go

@@ -7,6 +7,7 @@ import (
 	"log/slog"
 	"sort"
 	"strings"
+	"sync"
 	"time"
 
 	"git.mchus.pro/mchus/quoteforge/internal/appmeta"
@@ -22,9 +23,10 @@ var ErrOffline = errors.New("database is offline")
 
 // Service handles synchronization between MariaDB and local SQLite
 type Service struct {
-	connMgr  *db.ConnectionManager
-	localDB  *localdb.LocalDB
-	directDB *gorm.DB
+	connMgr     *db.ConnectionManager
+	localDB     *localdb.LocalDB
+	directDB    *gorm.DB
+	pricelistMu sync.Mutex // prevents concurrent pricelist syncs
 }
 
 // NewService creates a new sync service
@@ -939,9 +941,15 @@ func (s *Service) GetPricelistForOffline(serverPricelistID uint) (*localdb.Local
 	return localPL, nil
 }
 
-// SyncPricelistsIfNeeded checks for new pricelists and syncs if needed
-// This should be called before creating a new configuration when online
+// SyncPricelistsIfNeeded checks for new pricelists and syncs if needed.
+// If a sync is already in progress, returns immediately without blocking.
 func (s *Service) SyncPricelistsIfNeeded() error {
+	if !s.pricelistMu.TryLock() {
+		slog.Debug("pricelist sync already in progress, skipping")
+		return nil
+	}
+	defer s.pricelistMu.Unlock()
+
 	needSync, err := s.NeedSync()
 	if err != nil {
 		slog.Warn("failed to check if sync needed", "error", err)