Add a list of existing custom backgrounds in Settings

Currently, when uploading a new custom background image (Settings -> Apperance), the image is stored in `/input/backgrounds`.

It would be useful to provide a dropdown to select previously uploaded backgrounds, in addition to the upload button, to enable easy switching between available backgrounds, perhaps with some default backgrounds that replace the current hard-coded bitmap (see #5243).

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-5455-Add-a-list-of-existing-custom-backgrounds-in-Settings-2696d73d365081088fcbc04a2c31f42c) by [Unito](https://www.unito.io)