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/tracks.ts

@@ -1,87 +1,48 @@
 import {
 	player,
-	queueIdForTrack,
 	toQueueItems,
 	type ApiTrackLike,
 	type QueueItem
 } from '$lib/player/player.svelte';
-import type { YouTubeSong } from '$lib/types';
 
-export type TrackTarget =
-	{ kind: 'youtube'; song: YouTubeSong } | { kind: 'local'; track: ApiTrackLike };
+export type TrackTarget = { track: ApiTrackLike };
 
 export function targetId(target: TrackTarget): string | number {
-	return target.kind === 'youtube' ? target.song.videoId : target.track.id;
+	return target.track.id;
 }
 
 export function targetTitle(target: TrackTarget): string {
-	return target.kind === 'youtube' ? target.song.title : target.track.title;
+	return target.track.title;
 }
 
 export function targetArtist(target: TrackTarget): string | undefined {
-	return (target.kind === 'youtube' ? target.song.artist : target.track.artist) ?? undefined;
+	return target.track.artist ?? undefined;
 }
 
 export function targetOwnerUserId(target: TrackTarget): string | number | undefined {
-	return target.kind === 'youtube' ? undefined : (target.track.ownerUserId ?? undefined);
+	return target.track.ownerUserId ?? undefined;
 }
 
 export function targetExplicit(target: TrackTarget): boolean {
-	return target.kind === 'youtube' ? target.song.isExplicit : (target.track.isExplicit ?? false);
-}
-
-export function targetDurationSeconds(target: TrackTarget): number | undefined {
-	return target.kind === 'youtube' ? target.song.durationSeconds : undefined;
-}
-
-export function targetCoverSrc(target: TrackTarget): string | undefined {
-	return target.kind === 'youtube' ? target.song.thumbnailUrl : undefined;
-}
-
-function queueIdForTarget(target: TrackTarget): string | number {
-	return target.kind === 'youtube' ? target.song.videoId : queueIdForTrack(target.track);
+	return target.track.isExplicit ?? false;
 }
 
 export function isTargetCurrent(target: TrackTarget): boolean {
-	return player.current.id === queueIdForTarget(target);
+	return player.current.id === target.track.id;
 }
 
 export function queueItemForTarget(target: TrackTarget): QueueItem {
-	return target.kind === 'youtube'
-		? {
-				id: target.song.videoId,
-				title: target.song.title,
-				artist: target.song.artist,
-				source: 'youtube',
-				coverUrl: target.song.thumbnailUrl,
-				explicit: target.song.isExplicit
-			}
-		: toQueueItems([target.track])[0];
+	return toQueueItems([target.track])[0];
 }
 
 export function targetForQueueItem(item: QueueItem): TrackTarget {
-	return item.source === 'youtube'
-		? {
-				kind: 'youtube',
-				song: {
-					videoId: String(item.id),
-					title: item.title,
-					artist: item.artist ?? '',
-					album: '',
-					durationSeconds: 0,
-					thumbnailUrl: item.coverUrl ?? '',
-					isExplicit: item.explicit ?? false
-				}
-			}
-		: {
-				kind: 'local',
-				track: {
-					id: item.id,
-					title: item.title,
-					artist: item.artist,
-					source: 'Local',
-					isExplicit: item.explicit,
-					ownerUserId: item.ownerUserId
-				}
-			};
+	return {
+		track: {
+			id: item.id,
+			title: item.title,
+			artist: item.artist,
+			isExplicit: item.explicit,
+			ownerUserId: item.ownerUserId
+		}
+	};
 }