Increased the UI cover art request size from 300px to 600px for sharper
images on high-DPI displays. Replaced BiLinear with CatmullRom (bicubic)
interpolation for higher quality image resizing. Extracted the hardcoded
size into a COVER_ART_SIZE constant in the frontend and consolidated
backend sizes into a CacheWarmerImageSizes slice. Removed the unused
UIThumbnailSize constant.
Signed-off-by: Deluan <deluan@navidrome.org>
