Add party player: YouTube queue with crossfading playback

Two frontends sharing one WebSocket-backed state on the server:

- Player: fullscreen poster background with a small frame (30% of the
  screen) that drifts diagonally and reflects off the edges. Two stacked
  YouTube players hand over with a 5s equal-power crossfade so there is
  never silence between tracks.
- Queue: add by URL, drag to reorder, delete, click to jump. Shows title
  and length per item, highlights what is playing, and tracks remaining
  vs. total playtime.

No API key is required — titles come from YouTube's oEmbed endpoint and
durations from a hidden muted probe player that cues each new video just
to read its length.

The fade is driven by both requestAnimationFrame and the interval tick:
a hidden or throttled tab stops serving frames entirely, which would
otherwise strand a handover half-faded with two tracks audible.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Tg15tLAuT6NrCE1ZxKVgDB

public/player/index.html

@@ -0,0 +1,36 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <title>Party Player</title>
+    <link rel="stylesheet" href="/player/player.css" />
+  </head>
+  <body>
+    <div id="backgrounds"></div>
+
+    <!-- #frameMover owns the motion transform and nothing else, so future
+         animations can nest their own transform layers without conflict. -->
+    <div id="frameMover">
+      <div id="frameSkin">
+        <div class="deck" data-deck="a"><div id="deck-a"></div></div>
+        <div class="deck" data-deck="b"><div id="deck-b"></div></div>
+      </div>
+    </div>
+
+    <!-- Offscreen, muted, 1x1: cues queued videos purely to read their length. -->
+    <div id="probe"><div id="probe-player"></div></div>
+
+    <div id="gate">
+      <div class="gate-inner">
+        <h1>Party Player</h1>
+        <p id="gate-hint">Loading queue…</p>
+        <button id="start" type="button" disabled>Start</button>
+      </div>
+    </div>
+
+    <div id="status" hidden>reconnecting…</div>
+
+    <script type="module" src="/player/player.js"></script>
+  </body>
+</html>