Show thumbnails of photos without having them stored locally

### Please agree to the following

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

### Summary

Show thumbnails of photos without having them stored locally

### Motivation

It's very helpful to see what the photo is before downloading it completely. That's why a preview/thumbnail is important. This is needed whenever you have slow internet connection or a slow server.

This option should be per-folder and make cryptomator

1. download the files in the background,
2. generate thumbnails,
3. store them locally (encrypted) and
4. delete the actual photos since storage is limited on mobile devices.

I know that this process takes a while but you will have thumbnails for future access, even without internet connection.

### Considered Alternatives

_No response_

### Anything else?

_No response_