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/Cover.svelte

@@ -7,7 +7,7 @@
 		trackId: string | number;
 		hue: number;
 		size?: 'small' | 'medium' | 'large';
-		src?: string;
+		src?: string | null;
 		alt?: string;
 		class?: string;
 		children?: Snippet;