Add folder sync option for Android (background sync to/from vault)

### Please agree to the following

- [x] I have searched [existing issues](https://github.com/cryptomator/android/issues?q=) for duplicates
- [x] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/android/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Add folder sync option for Android (background sync to/from vault) as a substitute for live drive.

### Motivation

This is for everyone wanting to work seemlessly with files. I know it's a massive request but currently it's amazing it has been made, now to get access to files

People can secure there devices so working with a local folder seems like the workaround to a live drive.

On Windows, Cryptomator works seamlessly because it mounts the vault as a live drive, allowing any app to read/write files directly.

On Android, this is not possible due to OS restrictions. Working inside the vault is limited—working with the files is not fluent.

Request: Add a folder sync feature for Android that allows a selected local folder to be automatically synced with a folder inside an unlocked vault.

Proposed behavior:

- User selects a local folder (e.g., /Documents/text-library) and a target vault folder.
- When the vault is unlocked, Cryptomator syncs changes in both directions.
- Configurable sync interval (e.g., every 5 minutes, hourly, or manually triggered).
- Set to run in the background as standard.

Why this helps:
For example, when working with texts and photos, I cannot work with, access, or search files in a fluent way. This feature would solve that. I can not work with libraries of files.

Thanks

### Considered Alternatives

_No response_

### Anything else?

_No response_