[Feature] Folder thumbnail user-assigned

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request.

• Yes

The feature

When using Immich with a large already structured external library the best way to browse through it is to use the Folder view (e.g. on an external device like an Apple TV).
It would be a nice feature if users could independently assign an image from this folder as a thumbnail. So, the library in a folder view will look more conscious and beautiful. Using the first or random asset from a folder doesn't often give you an accurate idea of the folder's content.
Other viewing apps will be able to use this user-assigned thumbnail in the Folder view.
This feature is realized in Synology Photos and such feature in Immich will be a good reason to completely switch to it.

Platform

• Server
• Web
• Mobile