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/components/player/TrackInfo.svelte

@@ -20,8 +20,6 @@
 			id: player.current.id,
 			title: player.current.title,
 			artist: player.current.artist,
-			coverUrl: player.current.coverUrl,
-			source: player.current.source,
 			explicit: player.current.explicit,
 			ownerUserId: player.current.ownerUserId
 		});
@@ -32,7 +30,6 @@
 	{#if player.current.id}
 		<Cover
 			trackId={player.current.id}
-			src={player.current.coverUrl}
 			size="small"
 			alt={player.current.title}
 			class="h-11 w-11 shrink-0 rounded-[11px] shadow-cover-xs md:h-12 md:w-12"