Add skeleton type definition for last.fm API

Thought the now playing part of the code was malformed but now that I've
written out the minimal structure and checked it against what the API
returns I can see it's all good.

The API is all horribly converted from XML to JSON.

frontend/src/ts/scrobblar.ts

···
+interface Scrobbles {
+  recenttracks: {
+    track: {
+      artist: { "#text": string };
+      image: { "#text": string }[];
+      name: string;
+      "@attr"?: { nowplaying: boolean };
+    }[];
+  };
+}
+
 async function pollNowListening() {
   const resp = await fetch("/scrobbles.json");
-  const data = await resp.json();
+  const data: Scrobbles = await resp.json();
   const track = data.recenttracks.track[0];
   const trackData = track["@attr"];
   const nowPlaying = trackData?.nowplaying ?? false;