fix: detect video output from data in Nodes 2.0 (#8943)

## Summary

- Fixes SaveWebM node showing "Error loading image" in Vue nodes mode
- Extracts `isAnimatedOutput`/`isVideoOutput` utility functions from
inline logic in `unsafeUpdatePreviews` so both the litegraph canvas
renderer and Vue nodes renderer can detect video output directly from
execution data
- Uses output-based detection in `imagePreviewStore.isImageOutputs` to
avoid applying image preview format conversion to video files

## Background

In Vue nodes mode, `nodeMedia` relied on `node.previewMediaType` to
determine if output is video. This property is only set via
`onDrawBackground` → `unsafeUpdatePreviews` in the litegraph canvas
path, which doesn't run in Vue nodes mode. This caused webm output to
render via `<img>` instead of `<video>`.

## Before


https://github.com/user-attachments/assets/36f8a033-0021-4351-8f82-d19e3faa80c2


## After


https://github.com/user-attachments/assets/6558d261-d70e-4968-9637-6c24532e23ac
## Test plan

- [x] `pnpm typecheck` passes
- [x] `pnpm lint` passes
- [x] `pnpm test:unit` passes (4500 tests)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8943-fix-detect-video-output-from-data-in-Vue-nodes-mode-30a6d73d365081e98e91d6d1dcc88785)
by [Unito](https://www.unito.io)

---------

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

src/stores/imagePreviewStore.ts

@@ -14,7 +14,7 @@ import { app } from '@/scripts/app'
 import { useExecutionStore } from '@/stores/executionStore'
 import type { NodeLocatorId } from '@/types/nodeIdentification'
 import { parseFilePath } from '@/utils/formatUtil'
-import { isVideoNode } from '@/utils/litegraphUtil'
+import { isAnimatedOutput, isVideoNode } from '@/utils/litegraphUtil'
 import {
   releaseSharedObjectUrl,
   retainSharedObjectUrl
@@ -83,7 +83,7 @@ export const useNodeOutputStore = defineStore('nodeOutput', () => {
     outputs: ExecutedWsMessage['output']
   ): boolean => {
     // If animated webp/png or video outputs, return false
-    if (node.animatedImages || isVideoNode(node)) return false
+    if (isAnimatedOutput(outputs) || isVideoNode(node)) return false
 
     // If no images, return false
     if (!outputs?.images?.length) return false