refactor(app-asset): migrate file upload to presigned URL and batch upload

- Replace FormData file upload with presigned URL two-step upload
- Add batch-upload contract for folder uploads (reduces N+M to 1+M requests)
- Remove deprecated createFile contract and useCreateAppAssetFile hook
- Remove checksum field from AppAssetNode and AppAssetTreeView types
- Add upload-to-presigned-url utility for direct storage uploads

web/app/components/workflow/skill/utils/tree-utils.ts

@@ -161,7 +161,6 @@ export function createDraftTreeNode(options: DraftTreeNodeOptions): AppAssetTree
     path: '',
     extension: '',
     size: 0,
-    checksum: '',
     children: [],
   }
 }