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/routes/api/playlists/[id]/cover/+server.ts

@@ -0,0 +1,25 @@
+import type { RequestHandler } from './$types';
+import { apiConfig } from '$lib/server/config';
+
+const ALLOWED = new Set(['small', 'medium', 'large']);
+
+export const GET: RequestHandler = async ({ params, url, fetch }) => {
+	const size = url.searchParams.get('size') ?? 'medium';
+	const safeSize = ALLOWED.has(size) ? size : 'medium';
+
+	const upstream = await fetch(
+		`${apiConfig.baseUrl}/playlists/${params.id}/cover?size=${safeSize}`
+	);
+
+	if (!upstream.ok) {
+		return new Response(null, { status: upstream.status });
+	}
+
+	return new Response(upstream.body, {
+		status: 200,
+		headers: {
+			'Content-Type': upstream.headers.get('content-type') ?? 'image/webp',
+			'Cache-Control': 'public, max-age=86400'
+		}
+	});
+};