Download current spread as a single merged image

[DenDen047]

Describe your suggested feature

When reading in double-page (spread) mode, I'd like to download the currently displayed spread as a single combined image. For example, if pages 4–5 are shown side by side, clicking a button would merge them into one image and trigger a download.

Currently I have to download each page individually and stitch them together manually using an external image editor, which is tedious.

Other details

There are two possible implementation approaches:

1. Client-side (Canvas API): Since the reader already has both page images loaded, they could be drawn onto a canvas and exported. This requires no backend changes.
2. Server-side API: A new endpoint (e.g. GET /api/v1/books/{id}/pages/spread?pages=4,5) that merges the images on the server and returns the combined result. This preserves original image quality and could be reused by other clients.

I'd be happy to work on a PR for either approach if this feature is welcome.

Acknowledgements

• I have searched the existing issues and this is a new ticket, NOT a duplicate or related to another open issue.
• I have written a short but informative title.
• I have updated the app to the latest version.
• I will fill out all of the requested information in this form.