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/routes/(app)/playlists/[id]/+page.server.ts

@@ -41,7 +41,7 @@ export const load: PageServerLoad = async ({ params, locals, url, fetch }) => {
 	const alreadyLinked = new Set(
 		tracks.filter((t) => t.source === 'YouTube' && t.externalId).map((t) => t.externalId)
 	);
-	const seeds = [...new Set(tracks.map((t) => t.artist).filter((a) => !!a))];
+	const seeds = [...new Set(tracks.map((t) => t.artist).filter((a): a is string => !!a))];
 	const seed = seeds.length > 0 ? shuffle(seeds)[0] : undefined;
 
 	const youtube = fetchYoutubeFiller(

web-player/src/routes/(app)/playlists/[id]/+page.svelte

@@ -66,7 +66,7 @@
 	<PlaylistForm
 		action="?/rename"
 		initialName={playlist.name}
-		initialDescription={playlist.description}
+		initialDescription={playlist.description ?? ''}
 		coverFallbackUrl="/api/playlists/{playlist.id}/cover?size=medium&v={encodeURIComponent(
 			playlist.updatedAt
 		)}"

web-player/src/routes/(app)/playlists/[id]/components/PlaylistHeader.svelte

@@ -11,7 +11,7 @@
 	interface Props {
 		id: string;
 		name: string;
-		description?: string;
+		description?: string | null;
 		trackIds: (string | number)[];
 		updatedAt: string;
 		playing: boolean;

web-player/src/routes/(app)/playlists/[id]/components/SuggestionsTable.svelte

@@ -71,7 +71,7 @@
 			: {
 					id: queueIdForTrack(item.track),
 					title: item.track.title,
-					artist: item.track.artist,
+					artist: item.track.artist ?? undefined,
 					source: 'local',
 					coverUrl: `/api/tracks/${item.track.id}/cover?size=small`
 				};