feat: implement background task system with notifications

- Added background task manager with goroutine execution and panic recovery
- Replaced SSE streaming with background task execution for:
  * Price recalculation (RecalculateAll)
  * Stock import (ImportStockLog)
  * Pricelist creation (CreateWithProgress)
- Implemented unified polling for task status and DB connection in frontend
- Added task indicator in top bar showing running tasks count
- Added toast notifications for task completion/error
- Tasks automatically cleaned up after 10 minutes
- Tasks show progress (0-100%) with descriptive messages
- Updated handler constructors to receive task manager
- Added API endpoints for task status (/api/tasks, /api/tasks/:id)

Fixes issue with SSE disconnection on slow connections during long-running operations

internal/localdb/components.go

@@ -43,7 +43,7 @@ func (l *LocalDB) SyncComponents(mariaDB *gorm.DB) (*ComponentSyncResult, error)
 		SELECT
 			l.lot_name,
 			l.lot_description,
-			COALESCE(c.code, SUBSTRING_INDEX(l.lot_name, '_', 1)) as category,
+			COALESCE(l.lot_category, c.code, SUBSTRING_INDEX(l.lot_name, '_', 1)) as category,
 			m.model,
 			m.current_price
 		FROM lot l
@@ -82,12 +82,6 @@ func (l *LocalDB) SyncComponents(mariaDB *gorm.DB) (*ComponentSyncResult, error)
 		category := ""
 		if row.Category != nil {
 			category = *row.Category
-		} else {
-			// Parse category from lot_name (e.g., "CPU_AMD_9654" -> "CPU")
-			parts := strings.SplitN(row.LotName, "_", 2)
-			if len(parts) >= 1 {
-				category = parts[0]
-			}
 		}
 
 		model := ""