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/recentlyPlayed.ts

@@ -16,8 +16,6 @@ export function mergeRecentlyPlayed(history: ApiTrackLike[], limit: number): Que
 			id,
 			title: t.title,
 			artist: t.artist,
-			source: t.source,
-			coverUrl: t.coverUrl,
 			explicit: t.explicit,
 			ownerUserId: t.ownerUserId
 		});
@@ -31,8 +29,6 @@ export function mergeRecentlyPlayed(history: ApiTrackLike[], limit: number): Que
 			id,
 			title: item.title,
 			artist: item.artist,
-			source: item.source ?? 'local',
-			coverUrl: item.coverUrl,
 			explicit: item.explicit,
 			ownerUserId: item.ownerUserId
 		});