fix(web-player): follow the API types where they are nullable

The checked-in schema was stale and typed artist, description and externalId
as always present, so the code trusted values the API returns as null. The
regenerated schema tells the truth; the API-shaped types now admit null and it
is normalised on the way into the player, where the state stays non-null.

web-player/src/lib/components/ui/TrackTitleCell.svelte

@@ -6,8 +6,8 @@
 	interface Props {
 		trackId: string | number;
 		title: string;
-		artist?: string;
-		coverSrc?: string;
+		artist?: string | null;
+		coverSrc?: string | null;
 		coverSize?: string;
 		hue: number;
 		explicit?: boolean;