Remove YouTube integration from web-player; unify Home track rows

Drops every YouTube-backed route, proxy and type (external
albums/tracks, thumbnail proxy, youtube.ts) and simplifies
player/track/mix types to the local-only catalog now that the
backend contract dropped Source/ExternalId. Genre tiles on Explore
and "Populares esta semana" on Home stay as deliberate client-only
placeholders per product decision, just without the YouTube backing.

Also finishes the design-system unification pass: Home's three
hand-rolled track rows (Continuar escuchando, Playlist destacada,
Populares esta semana) now reuse the shared TrackTitleCell component
instead of duplicating cover+title+artist markup, and the orphaned
Rail.svelte (no importers) is removed.

web-player/src/lib/mixes.ts

@@ -5,33 +5,15 @@ export type Mix = components['schemas']['MixApplicationResponse'];
 export type MixItem = components['schemas']['MixItemApplicationResponse'];
 
 export function mixItemKey(item: MixItem): string {
-	return item.source === 'YouTube' ? `yt:${item.videoId}` : `mf:${item.trackId}`;
+	return `mf:${item.trackId}`;
 }
 
 export function mixItemTarget(item: MixItem): TrackTarget {
-	if (item.source === 'YouTube') {
-		return {
-			kind: 'youtube',
-			song: {
-				videoId: item.videoId ?? '',
-				title: item.title,
-				artist: item.artist ?? '',
-				album: '',
-				durationSeconds: Number(item.durationSeconds),
-				thumbnailUrl: item.thumbnailUrl ?? '',
-				isExplicit: item.isExplicit
-			}
-		};
-	}
-
 	return {
-		kind: 'local',
 		track: {
-			id: item.trackId ?? '',
+			id: item.trackId,
 			title: item.title,
-			artist: item.artist,
-			source: 'Local',
-			isExplicit: item.isExplicit
+			artist: item.artist
 		}
 	};
 }