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)/albums/[id]/+page.server.ts

@@ -38,9 +38,7 @@ export const load: PageServerLoad = async ({ params, locals, url, fetch, parent
 
 	const tracks = tracksRes.data?.items ?? [];
 	const inAlbum = new Set(tracks.map((track) => track.id));
-	const library = (libraryRes?.data?.items ?? []).filter(
-		(track) => track.source === 'Local' && !inAlbum.has(track.id)
-	);
+	const library = (libraryRes?.data?.items ?? []).filter((track) => !inAlbum.has(track.id));
 
 	const isOwner = user !== null && String(album.ownerUserId) === user.sub;