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/Cover.svelte

@@ -1,6 +1,5 @@
 <script lang="ts">
 	import type { Snippet } from 'svelte';
-	import { thumbnailSrc } from '$lib/thumbnails';
 
 	interface Props {
 		trackId: string | number;
@@ -24,9 +23,7 @@
 
 	let failed = $state(false);
 
-	const imageSrc = $derived(
-		src ? (thumbnailSrc(src, size) ?? src) : `/api/tracks/${trackId}/cover?size=${size}`
-	);
+	const imageSrc = $derived(src ?? `/api/tracks/${trackId}/cover?size=${size}`);
 
 	$effect(() => {
 		imageSrc;

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}

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

@@ -1,29 +0,0 @@
-<script lang="ts">
-	import type { Snippet } from 'svelte';
-
-	interface Props {
-		class?: string;
-		children: Snippet;
-	}
-
-	let { class: klass = '', children }: Props = $props();
-
-	let track: HTMLDivElement | undefined = $state();
-
-	export function scrollByPage(direction: 1 | -1) {
-		track?.scrollBy({ left: direction * track.clientWidth * 0.85, behavior: 'smooth' });
-	}
-</script>
-
-<div bind:this={track} class="no-scrollbar -mx-1 flex gap-4 overflow-x-auto px-1 pb-2 {klass}">
-	{@render children()}
-</div>
-
-<style>
-	.no-scrollbar {
-		scrollbar-width: none;
-	}
-	.no-scrollbar::-webkit-scrollbar {
-		display: none;
-	}
-</style>