feat: add download button to audio preview player (#8628)

## Summary
- Adds a download icon button to the `AudioPreviewPlayer` widget for
PreviewAudio and SaveAudio nodes
- Reuses the existing `downloadFile` utility (same as video download)
- Button appears inline next to volume/options controls, matching the
player's existing UI style

## Test plan
- [x] Add a PreviewAudio or SaveAudio node, run a workflow that produces
audio output
- [x] Verify the download icon appears in the audio player controls
- [x] Click the download button and confirm the audio file downloads
correctly
- [x] Verify the button does not appear when no audio is loaded


https://github.com/user-attachments/assets/7fb2df16-82a6-40aa-a938-aed57032e30b

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8628-feat-add-download-button-to-audio-preview-player-2fe6d73d365081e3997fc45d3bb8cffc)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

src/locales/en/main.json

@@ -10,6 +10,7 @@
     "download": "Download",
     "downloadImage": "Download image",
     "downloadVideo": "Download video",
+    "downloadAudio": "Download audio",
     "editOrMaskImage": "Edit or mask image",
     "editImage": "Edit image",
     "decrement": "Decrement",