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.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web-player/src/lib/components/ui/MixArt.svelte

@@ -15,21 +15,11 @@
 {#if tiles.length >= 4}
 	<div class="grid grid-cols-2 grid-rows-2 overflow-hidden {klass}">
 		{#each tiles as item (mixItemKey(item))}
-			<Cover
-				trackId={item.trackId ?? item.videoId ?? ''}
-				src={item.thumbnailUrl}
-				size="small"
-				class="h-full w-full"
-			/>
+			<Cover trackId={item.trackId} size="small" class="h-full w-full" />
 		{/each}
 	</div>
 {:else if tiles.length > 0}
-	<Cover
-		trackId={tiles[0].trackId ?? tiles[0].videoId ?? ''}
-		src={tiles[0].thumbnailUrl}
-		size="medium"
-		class={klass}
-	/>
+	<Cover trackId={tiles[0].trackId} size="medium" class={klass} />
 {:else}
 	<div class="bg-surface {klass}"></div>
 {/if}