fix: disable job asset actions when preview output is missing (#8700)

### Motivation
- Prevent context-menu actions from operating on stale or non-existent
assets when a completed job has no `previewOutput`, since `Inspect
asset`, `Add to current workflow`, and `Download` should be inactive in
that case.
- Also ensure the `Inspect asset` entry is disabled when the optional
inspect callback is not provided to avoid unexpected behavior.

### Description
- Added an optional `disabled?: boolean` field to the `MenuEntry` type
returned by `useJobMenu` and computed `hasPreviewAsset` to detect when
`taskRef.previewOutput` is present.
- Mark `inspect-asset`, `add-to-current`, and `download` entries as
`disabled` when the preview is missing (and also when the inspect
callback is missing for `inspect-asset`), and keep `delete` omitted when
no preview exists.
- Updated `JobContextMenu.vue` to pass `:disabled="entry.disabled"` to
the rendered `Button` and to short-circuit the action emit when an entry
is disabled.
- Expanded `useJobMenu` unit tests to assert enabled states when a
preview exists and disabled states when preview or inspect handler is
missing.

### Testing
- Ran `pnpm vitest src/composables/queue/useJobMenu.test.ts` and all
tests passed (36 tests).
- Ran `pnpm lint` and the linter reported no warnings or errors.
- Ran `pnpm typecheck` (`vue-tsc --noEmit`) and type checking completed
without errors.

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_e_69864ed56e408330b88c3e9def1b5fb5)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8700-fix-disable-job-asset-actions-when-preview-output-is-missing-2ff6d73d365081b8b72ccadf0ae43e9d)
by [Unito](https://www.unito.io)

src/composables/queue/useJobMenu.test.ts

@@ -743,6 +743,17 @@ describe('useJobMenu', () => {
       'd3',
       'delete'
     ])
+
+    expect(
+      findActionEntry(jobMenuEntries.value, 'inspect-asset')?.disabled
+    ).toBe(false)
+    expect(
+      findActionEntry(jobMenuEntries.value, 'add-to-current')?.disabled
+    ).toBe(false)
+    expect(findActionEntry(jobMenuEntries.value, 'download')?.disabled).toBe(
+      false
+    )
+
     const inspectEntry = findActionEntry(jobMenuEntries.value, 'inspect-asset')
     await inspectEntry?.onClick?.()
     expect(inspectSpy).toHaveBeenCalledWith(currentItem.value)
@@ -760,6 +771,7 @@ describe('useJobMenu', () => {
     await nextTick()
     const inspectEntry = findActionEntry(jobMenuEntries.value, 'inspect-asset')
     expect(inspectEntry?.onClick).toBeUndefined()
+    expect(inspectEntry?.disabled).toBe(true)
   })
 
   it('omits delete asset entry when no preview exists', async () => {
@@ -767,6 +779,15 @@ describe('useJobMenu', () => {
     setCurrentItem(createJobItem({ state: 'completed', taskRef: {} }))
 
     await nextTick()
+    expect(
+      findActionEntry(jobMenuEntries.value, 'inspect-asset')?.disabled
+    ).toBe(true)
+    expect(
+      findActionEntry(jobMenuEntries.value, 'add-to-current')?.disabled
+    ).toBe(true)
+    expect(findActionEntry(jobMenuEntries.value, 'download')?.disabled).toBe(
+      true
+    )
     expect(jobMenuEntries.value.some((entry) => entry.key === 'delete')).toBe(
       false
     )

src/composables/queue/useJobMenu.ts

@@ -29,6 +29,7 @@ export type MenuEntry =
       key: string
       label: string
       icon?: string
+      disabled?: boolean
       onClick?: () => void | Promise<void>
     }
   | { kind: 'divider'; key: string }
@@ -239,13 +240,14 @@ export function useJobMenu(
     const item = currentMenuItem()
     const state = item?.state
     if (!state) return []
-    const hasDeletableAsset = !!item?.taskRef?.previewOutput
+    const hasPreviewAsset = !!item?.taskRef?.previewOutput
     if (state === 'completed') {
       return [
         {
           key: 'inspect-asset',
           label: st('queue.jobMenu.inspectAsset', 'Inspect asset'),
           icon: 'icon-[lucide--zoom-in]',
+          disabled: !hasPreviewAsset || !onInspectAsset,
           onClick: onInspectAsset
             ? () => {
                 const item = currentMenuItem()
@@ -260,12 +262,14 @@ export function useJobMenu(
             'Add to current workflow'
           ),
           icon: 'icon-[comfy--node]',
+          disabled: !hasPreviewAsset,
           onClick: addOutputLoaderNode
         },
         {
           key: 'download',
           label: st('queue.jobMenu.download', 'Download'),
           icon: 'icon-[lucide--download]',
+          disabled: !hasPreviewAsset,
           onClick: downloadPreviewAsset
         },
         { kind: 'divider', key: 'd1' },
@@ -289,7 +293,7 @@ export function useJobMenu(
           onClick: copyJobId
         },
         { kind: 'divider', key: 'd3' },
-        ...(hasDeletableAsset
+        ...(hasPreviewAsset
           ? [
               {
                 key: 'delete',