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/navigation.svelte.ts

@@ -7,7 +7,6 @@ const LABELS: [RegExp, string][] = [
 	[/^\/explore\/?$/, 'Volver a explorar'],
 	[/^\/playlists\/[^/]+/, 'Volver a la lista'],
 	[/^\/playlists\/?$/, 'Volver a tus listas'],
-	[/^\/albums\/youtube\/[^/]+/, 'Volver al álbum'],
 	[/^\/albums\/[^/]+/, 'Volver al álbum'],
 	[/^\/albums\/?$/, 'Volver a tus álbumes'],
 	[/^\/library\/?$/, 'Volver a tus canciones'],