feat: add KSampler live previews to assets sidebar jobs (#8723)

## Summary
Show live KSampler previews on active job cards/list items in the Assets
sidebar, while preserving existing fallback behavior.

## Changes
- **What**:
- Added a prompt-scoped job preview store (`jobPreviewStore`) gated by
`Comfy.Execution.PreviewMethod`.
- Wired `b_preview_with_metadata` handling to map previews by
`promptId`.
- Extended queue job view model with `livePreviewUrl` and consumed it in
both sidebar list and grid active job UIs.
  - Cleared prompt previews on execution reset.
- Added ref-counted shared blob URL lifecycle utility (`objectUrlUtil`)
and updated preview stores to retain/release shared URLs so each preview
event creates one object URL.
- Added/updated unit coverage in `useJobList.test.ts` for preview
enable/disable mapping.

## Review Focus
- Object URL lifecycle correctness across node previews and job previews
(retain/release behavior).
- Preview gating behavior when `Comfy.Execution.PreviewMethod` is
`none`.
- Active job UI fallback behavior (`livePreviewUrl` -> `iconImageUrl`).

## Screenshots (if applicable)
<img width="808" height="614" alt="image"
src="https://github.com/user-attachments/assets/37c66eb2-8c28-4eb4-bb86-5679cb77d740"
/>
<img width="775" height="345" alt="image"
src="https://github.com/user-attachments/assets/aa420642-b0d4-4ae6-b94a-e7934b5df9d6"
/>


┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8723-feat-add-KSampler-live-previews-to-assets-sidebar-jobs-3006d73d365081aeb81dd8279bf99f94)
by [Unito](https://www.unito.io)

src/composables/queue/useJobList.test.ts

@@ -134,6 +134,25 @@ vi.mock('@/stores/executionStore', () => ({
   }
 }))
 
+let jobPreviewStoreMock: {
+  previewsByPromptId: Record<string, string>
+  isPreviewEnabled: boolean
+}
+const ensureJobPreviewStore = () => {
+  if (!jobPreviewStoreMock) {
+    jobPreviewStoreMock = reactive({
+      previewsByPromptId: {} as Record<string, string>,
+      isPreviewEnabled: true
+    })
+  }
+  return jobPreviewStoreMock
+}
+vi.mock('@/stores/jobPreviewStore', () => ({
+  useJobPreviewStore: () => {
+    return ensureJobPreviewStore()
+  }
+}))
+
 let workflowStoreMock: {
   activeWorkflow: null | { activeState?: { id?: string } }
 }
@@ -186,6 +205,10 @@ const resetStores = () => {
   executionStore.activePromptId = null
   executionStore.executingNode = null
 
+  const jobPreviewStore = ensureJobPreviewStore()
+  jobPreviewStore.previewsByPromptId = {}
+  jobPreviewStore.isPreviewEnabled = true
+
   const workflowStore = ensureWorkflowStore()
   workflowStore.activeWorkflow = null
 
@@ -437,6 +460,44 @@ describe('useJobList', () => {
     expect(otherJob.computeHours).toBeCloseTo(1)
   })
 
+  it('assigns preview urls for running jobs when previews enabled', async () => {
+    queueStoreMock.runningTasks = [
+      createTask({
+        promptId: 'live-preview',
+        queueIndex: 1,
+        mockState: 'running'
+      })
+    ]
+    jobPreviewStoreMock.previewsByPromptId = {
+      'live-preview': 'blob:preview-url'
+    }
+    jobPreviewStoreMock.isPreviewEnabled = true
+
+    const { jobItems } = initComposable()
+    await flush()
+
+    expect(jobItems.value[0].iconImageUrl).toBe('blob:preview-url')
+  })
+
+  it('omits preview urls when previews are disabled', async () => {
+    queueStoreMock.runningTasks = [
+      createTask({
+        promptId: 'disabled-preview',
+        queueIndex: 1,
+        mockState: 'running'
+      })
+    ]
+    jobPreviewStoreMock.previewsByPromptId = {
+      'disabled-preview': 'blob:preview-url'
+    }
+    jobPreviewStoreMock.isPreviewEnabled = false
+
+    const { jobItems } = initComposable()
+    await flush()
+
+    expect(jobItems.value[0].iconImageUrl).toBeUndefined()
+  })
+
   it('derives current node name from execution store fallbacks', async () => {
     const instance = initComposable()
     await flush()