Controlled Document PDF preview fails with embedded images using platform:// URLs

## Summary

Printing a Controlled Document containing embedded images produces a PDF object, but Huly displays `Failed to preview` and the generated file cannot be downloaded.

Regular Huly Documents containing embedded images print correctly. The issue appears specific to `documents:class:ControlledDocument`.

## Environment

- Deployment: Huly self-hosted
- Huly version: `v0.7.426`
- Print image: `hardcoreeng/print:v0.7.426`
- Browser: Chrome 151 on Windows
- Document class: `documents:class:ControlledDocument`

The print service is configured with `FRONT_URL`, `ACCOUNTS_URL`, `STORAGE_CONFIG`, and `PRINT_URL` according to the self-hosting documentation.

## Steps to reproduce

1. Create or open a Controlled Document.
2. Insert an image into its content.
3. Select the PDF print/export action.
4. Wait for PDF generation.
5. Attempt to preview or download the generated PDF.

## Expected behavior

The Controlled Document should be exported to a downloadable PDF containing its embedded images, as happens with regular Huly Documents.

## Actual behavior

- The print endpoint returns HTTP 200.
- Huly displays `Failed to preview`.
- The generated PDF cannot be downloaded.
- The print service reports failed requests for embedded images.

## Relevant log

```text
requestfailed
errorText: net::ERR_UNKNOWN_URL_SCHEME
url: platform://platform/files/workspace/?file=
```

The same image is available during normal document viewing through the `/files/...` HTTP endpoint.

## Additional observations

- Controlled Documents without embedded images can be printed.
- Regular Huly Documents with embedded images can be printed.
- The affected images were copied from another Huly document.
- The images exist in Huly storage and display normally in the Controlled Document.
- No guest tokens, authentication errors, storage errors, or HTTP 5xx responses were observed.

## Suspected cause

The guest rendering path used to print a Controlled Document leaves embedded image sources in Huly's internal format:

```text
platform://platform/files/workspace/?file=
```

Headless Chromium does not support the `platform://` scheme.

The Controlled Document guest renderer may need to resolve these references to authenticated HTTP(S) `/files/...` URLs before Puppeteer calls `page.pdf()`.

## Possible fix

Resolve internal file references in the Controlled Document guest renderer, or replace `platform://platform/files/...` image sources with guest-accessible HTTP(S) URLs before PDF rendering.

## Security note

All workspace IDs, file IDs, document content, domains, and guest tokens have been redacted.

Start with the Controlled Document guest rendering path and the print endpoint, then trace how embedded image sources reach Puppeteer before page.pdf(). Compare the platform:// URL with the working /files/... HTTP endpoint. Done means Controlled Documents with embedded images produce a previewable, downloadable PDF while regular Documents continue to work.

Written by the indexing model from the issue text.