Revert "remove thumbnail cache"

This reverts commit e0db4931f1.

app/models/cache.ts

@@ -2,12 +2,16 @@ import { Album, Playlist, Artist, Song } from './library'
 
 export enum CacheItemType {
   coverArt = 'coverArt',
+  coverArtThumb = 'coverArtThumb',
   artistArt = 'artistArt',
+  artistArtThumb = 'artistArtThumb',
   song = 'song',
 }
 
 export type CacheItemTypeKey = keyof typeof CacheItemType
 
+export type CacheImageSize = 'thumbnail' | 'original'
+
 export type CacheFile = {
   path: string
   date: number