refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

web-player/src/lib/server/image.ts

@@ -0,0 +1,26 @@
+import { apiConfig } from '$lib/server/config';
+
+const ALLOWED_SIZES = new Set(['small', 'medium', 'large']);
+
+export async function proxyCoverImage(
+	fetch: typeof globalThis.fetch,
+	kind: 'playlists' | 'tracks',
+	id: string,
+	size: string | null
+): Promise<Response> {
+	const safeSize = size && ALLOWED_SIZES.has(size) ? size : 'medium';
+
+	const upstream = await fetch(`${apiConfig.baseUrl}/${kind}/${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'
+		}
+	});
+}