Consolidate image upload implementations into shared service

Creates core uploadService to eliminate ~60-70 LOC duplication across multiple implementations.

Changes:
- Add src/platform/assets/services/uploadService.ts with uploadMedia() and uploadMediaBatch()
- Refactor Load3dUtils to use uploadService (eliminates 50+ LOC)
- Refactor WidgetSelectDropdown to use uploadService (eliminates 20+ LOC)
- Add comprehensive unit tests for uploadService
- Maintain backward compatibility for all existing APIs

Benefits:
- Single source of truth for upload logic
- Consistent error handling
- Type-safe interfaces
- Easier to test and maintain

src/composables/useLoad3d.ts

@@ -388,8 +388,10 @@ export const useLoad3d = (nodeOrRef: MaybeRef<LGraphNode | null>) => {
       : '3d'
 
     const uploadedPath = await Load3dUtils.uploadFile(file, subfolder)
-    sceneConfig.value.backgroundImage = uploadedPath
-    await load3d?.setBackgroundImage(uploadedPath)
+    if (uploadedPath) {
+      sceneConfig.value.backgroundImage = uploadedPath
+      await load3d?.setBackgroundImage(uploadedPath)
+    }
   }
 
   const handleExportModel = async (format: string) => {