feat(ui): increase cover art size to 600px and use CatmullRom scaling

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>

core/artwork/cache_warmer_test.go

@@ -176,13 +176,13 @@ var _ = Describe("CacheWarmer", func() {
 			}).Should(Equal(0))
 		})
 
-		It("pre-caches both UICoverArtSize and UIThumbnailSize", func() {
+		It("pre-caches UICoverArtSize", func() {
 			cw := NewCacheWarmer(aw, fc).(*cacheWarmer)
 			cw.PreCache(model.MustParseArtworkID("al-1"))
 
 			Eventually(func() []int {
 				return aw.getCachedSizes()
-			}).Should(ContainElements(consts.UICoverArtSize, consts.UIThumbnailSize))
+			}).Should(ContainElements(consts.UICoverArtSize))
 		})
 	})
 })