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>

browser_tests/assets/widgets/save_image_and_animated_webp.json

@@ -0,0 +1,86 @@
+{
+  "id": "save-image-and-webm-test",
+  "revision": 0,
+  "last_node_id": 12,
+  "last_link_id": 2,
+  "nodes": [
+    {
+      "id": 10,
+      "type": "LoadImage",
+      "pos": [50, 100],
+      "size": [315, 314],
+      "flags": {},
+      "order": 0,
+      "mode": 0,
+      "inputs": [],
+      "outputs": [
+        {
+          "name": "IMAGE",
+          "type": "IMAGE",
+          "links": [1, 2]
+        },
+        {
+          "name": "MASK",
+          "type": "MASK",
+          "links": null
+        }
+      ],
+      "properties": {
+        "Node name for S&R": "LoadImage"
+      },
+      "widgets_values": ["example.png", "image"]
+    },
+    {
+      "id": 11,
+      "type": "SaveImage",
+      "pos": [450, 100],
+      "size": [210, 270],
+      "flags": {},
+      "order": 1,
+      "mode": 0,
+      "inputs": [
+        {
+          "name": "images",
+          "type": "IMAGE",
+          "link": 1
+        }
+      ],
+      "outputs": [],
+      "properties": {},
+      "widgets_values": ["ComfyUI"]
+    },
+    {
+      "id": 12,
+      "type": "SaveWEBM",
+      "pos": [450, 450],
+      "size": [210, 368],
+      "flags": {},
+      "order": 2,
+      "mode": 0,
+      "inputs": [
+        {
+          "name": "images",
+          "type": "IMAGE",
+          "link": 2
+        }
+      ],
+      "outputs": [],
+      "properties": {},
+      "widgets_values": ["ComfyUI", "vp9", 6, 32]
+    }
+  ],
+  "links": [
+    [1, 10, 0, 11, 0, "IMAGE"],
+    [2, 10, 0, 12, 0, "IMAGE"]
+  ],
+  "groups": [],
+  "config": {},
+  "extra": {
+    "frontendVersion": "1.17.0",
+    "ds": {
+      "offset": [0, 0],
+      "scale": 1
+    }
+  },
+  "version": 0.4
+}

browser_tests/tests/saveImageAndWebp.spec.ts

@@ -0,0 +1,42 @@
+import { expect } from '@playwright/test'
+
+import { comfyPageFixture as test } from '../fixtures/ComfyPage'
+
+test.describe(
+  'Save Image and WEBM preview',
+  { tag: ['@screenshot', '@widget'] },
+  () => {
+    test.beforeEach(async ({ comfyPage }) => {
+      await comfyPage.settings.setSetting('Comfy.VueNodes.Enabled', true)
+      await comfyPage.vueNodes.waitForNodes()
+    })
+
+    test('Can preview both SaveImage and SaveWEBM outputs', async ({
+      comfyPage
+    }) => {
+      await comfyPage.workflow.loadWorkflow(
+        'widgets/save_image_and_animated_webp'
+      )
+      await comfyPage.vueNodes.waitForNodes()
+
+      await comfyPage.runButton.click()
+
+      const saveImageNode = comfyPage.vueNodes.getNodeByTitle('Save Image')
+      const saveWebmNode = comfyPage.vueNodes.getNodeByTitle('SaveWEBM')
+
+      // Wait for SaveImage to render an img inside .image-preview
+      await expect(saveImageNode.locator('.image-preview img')).toBeVisible({
+        timeout: 30000
+      })
+
+      // Wait for SaveWEBM to render a video inside .video-preview
+      await expect(saveWebmNode.locator('.video-preview video')).toBeVisible({
+        timeout: 30000
+      })
+
+      await expect(comfyPage.page).toHaveScreenshot(
+        'save-image-and-webm-preview.png'
+      )
+    })
+  }
+)