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;