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/services/litegraphService.ts

@@ -56,8 +56,10 @@ import { useRightSidePanelStore } from '@/stores/workspace/rightSidePanelStore'
 import { useWidgetStore } from '@/stores/widgetStore'
 import { normalizeI18nKey } from '@/utils/formatUtil'
 import {
+  isAnimatedOutput,
   isImageNode,
   isVideoNode,
+  isVideoOutput,
   migrateWidgetsValues
 } from '@/utils/litegraphUtil'
 import { getOrderedInputSpecs } from '@/workbench/utils/nodeDefOrderingUtil'
@@ -753,17 +755,9 @@ export const useLitegraphService = () => {
     if (isNewOutput) this.images = output.images
 
     if (isNewOutput || isNewPreview) {
-      this.animatedImages = output?.animated?.find(Boolean)
+      this.animatedImages = isAnimatedOutput(output)
 
-      const isAnimatedWebp =
-        this.animatedImages &&
-        output?.images?.some((img) => img.filename?.includes('webp'))
-      const isAnimatedPng =
-        this.animatedImages &&
-        output?.images?.some((img) => img.filename?.includes('png'))
-      const isVideo =
-        (this.animatedImages && !isAnimatedWebp && !isAnimatedPng) ||
-        isVideoNode(this)
+      const isVideo = isVideoOutput(output) || isVideoNode(this)
       if (isVideo) {
         useNodeVideo(this, callback).showPreview()
       } else {