🗂📥 Donwload folders as .zip from mobile app

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### Is your feature request related to a problem? Please describe.

I'd love to be able to download a full folder as a ZIP archive directly from the mobile app (like it’s possible in the web UI).
This would make it much easier to:

- Download multiple files at once.
- Share full folder structures (outside Nextcloud).
- Prepare data for offline use or backup.

Currently, downloading a folder requires selecting all files inside (manually), which is tedious especially on mobile, or syncing the folder, which is not the thing I want. The option to download a folder as ZIP is missing completely.

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### Describe the solution you'd like

- Add a **"Download as ZIP"** option in the mobile app for folders.
- When tapping it, the app should:
- Trigger the server-side ZIP creation (as in web UI).
- Download the resulting archive.
- Save it in the Downloads or Files section.

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### Describe alternatives you've considered

- Synchronizing the whole folder – not the right solution, as I want to download the folder, but not sync
- Using the web version of Nextcloud on mobile – possible, but clunky.

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### Additional context

This feature already exists in the **Nextcloud web interface**, where a folder can be zipped and downloaded.

Having the same option in the mobile app would streamline workflows and bring mobile parity with the desktop/web experience.

Thanks for your great work! 🙌