refactor: rename imagePreviewStore to nodeOutputStore (#9416)

## Summary

Rename `imagePreviewStore.ts` → `nodeOutputStore.ts` to match the store
it houses (`useNodeOutputStore`, Pinia ID `nodeOutput`).

## Changes

- **What**: Rename file + test file, update all 21 import paths, mock
paths, and describe labels
- **Breaking**: None — exported symbol (`useNodeOutputStore`) and Pinia
store ID (`nodeOutput`) are unchanged

## Custom Node Ecosystem Audit

Searched the ComfyUI custom node ecosystem for `imagePreviewStore` and
`useNodeOutputStore`:
- **Not part of the public API** — neither filename nor export appear in
`comfyui_frontend_package` or `vite.types.config.mts`
- **1 external repo found:** `wallen0322/ComfyUI-AE-Animation` —
contains a full fork of the frontend source tree; it copies the file
internally and does not import from the published package. **No
breakage.**
- **No custom nodes import this store via the extension API.** This is a
safe internal-only rename.

## Review Focus

Pure mechanical rename — no logic changes. Verify no stale
`imagePreviewStore` references remain.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9416-refactor-rename-imagePreviewStore-to-nodeOutputStore-31a6d73d3650816086c5e62959861ddb)
by [Unito](https://www.unito.io)

Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/renderer/extensions/vueNodes/widgets/composables/useImageUploadWidget.ts

@@ -6,7 +6,7 @@ import type { IComboWidget } from '@/lib/litegraph/src/types/widgets'
 import type { ResultItemType } from '@/schemas/apiSchema'
 import type { InputSpec } from '@/schemas/nodeDefSchema'
 import type { ComfyWidgetConstructor } from '@/scripts/widgets'
-import { useNodeOutputStore } from '@/stores/imagePreviewStore'
+import { useNodeOutputStore } from '@/stores/nodeOutputStore'
 import { isImageUploadInput } from '@/types/nodeDefAugmentation'
 import { createAnnotatedPath } from '@/utils/createAnnotatedPath'
 import { addToComboValues } from '@/utils/litegraphUtil'