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/routes/(app)/liked/+page.svelte

@@ -23,8 +23,6 @@
 			id: t.id,
 			title: t.title,
 			artist: t.artist,
-			source: t.source,
-			coverUrl: t.coverUrl,
 			explicit: t.explicit
 		}))
 	);
@@ -97,7 +95,6 @@
 						title={track.title}
 						artist={track.artist}
 						ownerUserId={track.ownerUserId}
-						coverSrc={track.coverUrl}
 						explicit={track.explicit}
 						{active}
 						onClick={() => playFrom(i)}