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/utils/litegraphUtil.ts

@@ -5,6 +5,7 @@ import type {
   LGraph,
   LGraphCanvas
 } from '@/lib/litegraph/src/litegraph'
+import type { ExecutedWsMessage } from '@/schemas/apiSchema'
 import {
   LGraphGroup,
   LGraphNode,
@@ -77,6 +78,32 @@ export function isVideoNode(node: LGraphNode | undefined): node is VideoNode {
   return node.previewMediaType === 'video' || !!node.videoContainer
 }
 
+/**
+ * Check if output data indicates animated content (animated webp/png or video).
+ */
+export function isAnimatedOutput(
+  output: ExecutedWsMessage['output'] | undefined
+): boolean {
+  return !!output?.animated?.find(Boolean)
+}
+
+/**
+ * Check if output data indicates video content (animated but not webp/png).
+ */
+export function isVideoOutput(
+  output: ExecutedWsMessage['output'] | undefined
+): boolean {
+  if (!isAnimatedOutput(output)) return false
+
+  const isAnimatedWebp = output?.images?.some((img) =>
+    img.filename?.endsWith('.webp')
+  )
+  const isAnimatedPng = output?.images?.some((img) =>
+    img.filename?.endsWith('.png')
+  )
+  return !isAnimatedWebp && !isAnimatedPng
+}
+
 export function isAudioNode(node: LGraphNode | undefined): boolean {
   return !!node && node.previewMediaType === 'audio'
 }