feat(frontend): redesign playlist pages with cover image support

Create and edit playlist modals now upload/preview a cover image
(reusing the presigned-upload flow already used for tracks) and share
identical layout and coloring. Adds a cover proxy route and wires
PlaylistArt to render a playlist's real cover before falling back to
the track mosaic/gradient.

Also reworks the playlist detail page: full-width borderless "Añadir
de tu biblioteca" table with listens count and click-to-play, drops
the empty-playlist placeholder message, and restyles the playlists
grid to match the rest of the app.

frontend/src/lib/components/ui/PlaylistArt.svelte

@@ -3,12 +3,14 @@
 	import { gradientForHue, HUES } from '$lib/theme/color';
 
 	interface Props {
+		playlistId?: string;
 		trackIds: (string | number)[];
 		hue: number;
+		size?: 'small' | 'medium' | 'large';
 		class?: string;
 	}
 
-	let { trackIds, hue, class: klass = '' }: Props = $props();
+	let { playlistId, trackIds, hue, size = 'medium', class: klass = '' }: Props = $props();
 
 	function hueFor(id: string | number) {
 		const text = String(id);
@@ -17,11 +19,23 @@
 		return HUES[hash % HUES.length];
 	}
 
+	let coverFailed = $state(false);
+
 	const mosaic = $derived(trackIds.length >= 4);
 	const single = $derived(trackIds.length >= 1 && trackIds.length < 4 ? trackIds[0] : null);
 </script>
 
-{#if mosaic}
+{#if playlistId && !coverFailed}
+	<div class="relative overflow-hidden {klass}" style="background:{gradientForHue(hue)}">
+		<img
+			src="/api/playlists/{playlistId}/cover?size={size}"
+			alt=""
+			loading="lazy"
+			onerror={() => (coverFailed = true)}
+			class="h-full w-full object-cover"
+		/>
+	</div>
+{:else if mosaic}
 	<div class="grid grid-cols-2 grid-rows-2 overflow-hidden {klass}">
 		{#each trackIds.slice(0, 4) as id (id)}
 			<Cover trackId={id} hue={hueFor(id)} size="small" class="h-full w-full" />