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/api/tracks/+server.ts

@@ -7,11 +7,10 @@ export const GET: RequestHandler = async ({ url, locals, fetch }) => {
 	const name = url.searchParams.get('name')?.trim() || undefined;
 	const pageNumber = Math.max(1, Number(url.searchParams.get('pageNumber')) || 1);
 	const pageSize = Math.max(1, Number(url.searchParams.get('pageSize')) || EXPLORE_PAGE_SIZE);
-	const youtubeContinuationToken = url.searchParams.get('youtubeContinuationToken') || undefined;
 
 	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
 	const result = await api.GET('/tracks', {
-		params: { query: { name, pageNumber, pageSize, youtubeContinuationToken } }
+		params: { query: { name, pageNumber, pageSize } }
 	});
 
 	return json(unwrapOrError(result, 'No se pudieron cargar las canciones.'));