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

@@ -1,6 +1,6 @@
 import type { PageServerLoad, Actions } from './$types';
 import { createApiClient, requireUser, unwrapOrError } from '$lib/server/api';
-import { addTrackAction, addYouTubeToPlaylistAction } from '$lib/server/playlistActions';
+import { addTrackAction } from '$lib/server/playlistActions';
 import { PLAYLIST_PICKER_PAGE_SIZE } from '$lib/config';
 
 export const load: PageServerLoad = async ({ params, locals, url, fetch }) => {
@@ -23,6 +23,5 @@ export const load: PageServerLoad = async ({ params, locals, url, fetch }) => {
 };
 
 export const actions: Actions = {
-	addTrack: addTrackAction,
-	addYouTubeToPlaylist: addYouTubeToPlaylistAction
+	addTrack: addTrackAction
 };

web-player/src/routes/(app)/mixes/[id]/+page.svelte

@@ -25,7 +25,6 @@
 		isTargetCurrent,
 		queueItemForTarget,
 		targetArtist,
-		targetCoverSrc,
 		targetId,
 		targetTitle
 	} from '$lib/tracks';
@@ -107,8 +106,6 @@
 					trackId={targetId(targets[i])}
 					title={targetTitle(targets[i])}
 					artist={targetArtist(targets[i])}
-					coverSrc={targetCoverSrc(targets[i])}
-					explicit={item.isExplicit}
 					{active}
 					onClick={() => playFrom(i)}
 				/>