Allow downloading files in read-only iframe embeds #307

When opening a document in a read-only iframe, download controls remain available and generate a file, but no download starts. This gives the impression that the controls are malfunctioning.

Steps to reproduce

1. Open a document inside an iframe using /editor?src=<document-url>&readonly=1&embed=1.

2. Select a download/export option in the File tab.

3. Observe that no file is downloaded, although the console reports:

   [OO] file stream received: example.xlsx (22394 bytes)
   

Expected behavior

Read-only mode should allow downloading without enabling editing:

• Keep download controls functional when downloading is permitted.
• Allow downloading to be configured independently of editing permissions.
• Hide or visibly disable download controls when downloading is unavailable.

Implementation context

In lib/onlyoffice/save-stream.ts, routeSavedFile() skips local downloads in embed mode and recommends using document:save from the parent page. However, requestSaveDocument() rejects that command when the document is read-only.

Removing embed=1 does not change this behavior because lib/embed-mode.ts automatically detects iframe embedding.

Could downloading be supported independently of editing in read-only embeds? Allowing local downloads through an explicit option, or providing an API command that permits exporting a read-only document, would both address the issue.