fix: drag-and-drop screenshot creates LoadImage node instead of showing error (#8886)

## Summary

Fix drag-and-drop of local screenshots onto the canvas failing to create
a LoadImage node.

## Changes

- **What**: Replace `_.isEmpty(workflowData)` check in `handleFile` with
a check for workflow-relevant keys (`workflow`, `prompt`, `parameters`,
`templates`). PNG screenshots often contain non-workflow `tEXt` metadata
(e.g. `Software`, `Creation Time`) which made `_.isEmpty()` return
`false`, skipping the LoadImage fallback and showing an error instead.

## Review Focus

The root cause is that `getPngMetadata` extracts all `tEXt`/`iTXt` PNG
chunks indiscriminately. Rather than filtering at the parser level
(which could break extensions relying on arbitrary metadata), the fix
checks for workflow-relevant keys before deciding whether to treat the
file as a workflow or a plain image.

Fixes #7752

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8886-fix-drag-and-drop-screenshot-creates-LoadImage-node-instead-of-showing-error-3086d73d3650817d86c5f1386aa041c2)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

src/scripts/app.test.ts

@@ -196,5 +196,25 @@ describe('ComfyApp', () => {
         mockNode
       )
     })
+
+    it('should handle image files with non-workflow metadata by creating LoadImage node', async () => {
+      vi.mocked(getWorkflowDataFromFile).mockResolvedValue({
+        Software: 'gnome-screenshot'
+      })
+
+      const mockNode = createMockNode()
+      vi.mocked(createNode).mockResolvedValue(mockNode)
+
+      const imageFile = createTestFile('screenshot.png', 'image/png')
+
+      await app.handleFile(imageFile)
+
+      expect(createNode).toHaveBeenCalledWith(mockCanvas, 'LoadImage')
+      expect(pasteImageNode).toHaveBeenCalledWith(
+        mockCanvas,
+        expect.any(DataTransferItemList),
+        mockNode
+      )
+    })
   })
 })