Add optional image hosting for non-public Navidrome instances

When Navidrome is behind a private network (e.g. Tailscale), Discord
cannot fetch artwork URLs. This adds optional integration with imgbb
(24h expiry, requires API key) and uguu.se (3h expiry, no key needed)
to upload cover art to a public host.

Closes #1

# Conflicts:
#	go.mod
#	go.sum

main.go

@@ -91,21 +91,6 @@ func getConfig() (clientID string, users map[string]string, err error) {
 	return clientID, users, nil
 }
 
-// getImageURL retrieves the track artwork URL.
-func getImageURL(trackID string) string {
-	artworkURL, err := host.ArtworkGetTrackUrl(trackID, 300)
-	if err != nil {
-		pdk.Log(pdk.LogWarn, fmt.Sprintf("Failed to get artwork URL: %v", err))
-		return ""
-	}
-
-	// Don't use localhost URLs
-	if strings.HasPrefix(artworkURL, "http://localhost") {
-		return ""
-	}
-	return artworkURL
-}
-
 // ============================================================================
 // Scrobbler Implementation
 // ============================================================================
@@ -163,7 +148,7 @@ func (p *discordPlugin) NowPlaying(input scrobbler.NowPlayingRequest) error {
 			End:   endTime,
 		},
 		Assets: activityAssets{
-			LargeImage: getImageURL(input.Track.ID),
+			LargeImage: getImageURL(input.Username, input.Track.ID),
 			LargeText:  input.Track.Album,
 		},
 	}); err != nil {