Add shared YouTube playlist resizing

server.js

@@ -38,6 +38,26 @@ function normalizeName(name) {
   return normalized.slice(0, 48) || "Guest";
 }
 
+function normalizePlaylist(playlist) {
+  if (!Array.isArray(playlist)) return [];
+
+  const seen = new Set();
+  return playlist
+    .map((item) => ({
+      videoId: String(item?.videoId || "").trim(),
+      title: String(item?.title || "").trim()
+    }))
+    .filter((item) => {
+      if (!/^[a-zA-Z0-9_-]{11}$/.test(item.videoId) || seen.has(item.videoId)) {
+        return false;
+      }
+      seen.add(item.videoId);
+      item.title ||= `YouTube video ${item.videoId}`;
+      return true;
+    })
+    .slice(0, 50);
+}
+
 function broadcast(roomId, senderId, event) {
   const room = rooms.get(roomId);
   if (!room) return;
@@ -157,16 +177,19 @@ const server = http.createServer(async (req, res) => {
       const roomId = url.pathname.split("/").at(-1) || defaultRoomId;
       const payload = await parseJson(req);
 
-      if (!payload.from || payload.type !== "watch-state" || !payload.videoId) {
-        sendJson(res, 400, { error: "Watch messages require from, type=watch-state, and videoId." });
+      if (!payload.from || payload.type !== "watch-state") {
+        sendJson(res, 400, { error: "Watch messages require from and type=watch-state." });
         return;
       }
 
+      const playlist = normalizePlaylist(payload.playlist);
+      const videoId = String(payload.videoId || "").trim();
       const watchState = {
         type: "watch-state",
         from: payload.from,
         name: normalizeName(payload.name),
-        videoId: String(payload.videoId).trim(),
+        videoId: playlist.some((item) => item.videoId === videoId) ? videoId : "",
+        playlist,
         position: Number(payload.position) || 0,
         playing: Boolean(payload.playing),
         updatedAt: Number(payload.updatedAt) || Date.now()