fix: preserve input asset previews across execution updates (#9123)

## Summary

Fix input asset previews (images/videos) disappearing from
LoadImage/LoadVideo nodes after execution and a browser tab switch.

## Changes

- **What**: Guard `setOutputsByLocatorId` in `imagePreviewStore` to
preserve existing input-type preview images (`type: 'input'`) when the
incoming execution output has no images. Execution outputs with actual
images still overwrite as expected.

## Review Focus

- The guard only applies when existing output is an input preview (`type
=== 'input'` for all images) AND incoming output has no images — this is
the exact scenario where execution clobbers upload widget previews.
- Root cause: execution results from the backend overwrite the upload
widget's synthetic preview for LoadImage/LoadVideo nodes (which produce
no output images). Combined with the deferred resize-observer
re-observation from PR #8805, returning to a hidden tab reads the
now-empty store entry.

src/stores/nodeOutputStore.ts

@@ -126,6 +126,22 @@ export const useNodeOutputStore = defineStore('nodeOutput', () => {
     })
   }
 
+  /**
+   * Check if an output contains input-type preview images (from upload widgets).
+   * These are synthetic previews set by LoadImage/LoadVideo widgets, not
+   * execution results from the backend.
+   */
+  function isInputPreviewOutput(
+    output: ExecutedWsMessage['output'] | ResultItem | undefined
+  ): boolean {
+    const images = (output as ExecutedWsMessage['output'] | undefined)?.images
+    return (
+      Array.isArray(images) &&
+      images.length > 0 &&
+      images.every((i) => i?.type === 'input')
+    )
+  }
+
   /**
    * Internal function to set outputs by NodeLocatorId.
    * Handles the merge logic when needed.
@@ -140,6 +156,26 @@ export const useNodeOutputStore = defineStore('nodeOutput', () => {
     // (e.g., two LoadImage nodes selecting the same image)
     if (outputs == null) return
 
+    // Preserve input preview images (from upload widgets) when execution
+    // sends outputs with no images. Without this guard, execution results
+    // overwrite the upload widget's preview, causing LoadImage/LoadVideo
+    // nodes to lose their preview after execution + tab switch.
+    // Note: intentional preview clears go through setNodeOutputs (widget
+    // path), not setNodeOutputsByExecutionId, so this guard does not
+    // interfere with user-initiated clears.
+    const incomingImages = (outputs as ExecutedWsMessage['output']).images
+    const hasIncomingImages =
+      Array.isArray(incomingImages) && incomingImages.length > 0
+    if (
+      !hasIncomingImages &&
+      isInputPreviewOutput(app.nodeOutputs[nodeLocatorId])
+    ) {
+      outputs = {
+        ...outputs,
+        images: app.nodeOutputs[nodeLocatorId].images
+      }
+    }
+
     if (options.merge) {
       const existingOutput = app.nodeOutputs[nodeLocatorId]
       if (existingOutput && outputs) {