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

README.md

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+# Party Player
+
+A YouTube playlist player for parties. Two browser windows run side by side:
+
+- **Player** (`http://localhost:3000/`) — fullscreen poster background with a
+  small video frame drifting diagonally and bouncing off the edges. Consecutive
+  videos overlap in a 5 second audio/video crossfade, so there is never silence
+  between tracks.
+- **Queue** (`http://localhost:3000/queue`) — add videos by URL, drag to
+  reorder, delete, and see how much playtime is left.
+
+## Run
+
+```sh
+npm install
+npm start
+```
+
+Put the player on the projector/TV screen (F11 for fullscreen) and open the
+queue on a laptop or phone. On the same network, other devices reach both pages
+at `http://<this-machine-ip>:3000`.
+
+**The player page opens behind a "Start" button.** Browsers block unmuted
+autoplay, so the first click is what unlocks audio. One click per session.
+
+## Notes
+
+- No YouTube API key is needed. Titles come from YouTube's public oEmbed
+  endpoint; durations are measured by a hidden muted player on the player page,
+  which is why a freshly added video shows `–:––` for a moment (and keeps
+  showing it while the player page is closed). Totals are prefixed with `≥`
+  whenever a length is still unknown.
+- The queue is saved to `queue.json` on every change and restored at startup.
+- Settings live in `config.js`: overlap length, frame size, drift speed, port,
+  and the selectable backgrounds.
+
+## Adding animations
+
+`#frameMover` carries only the motion transform (see `public/player/motion.js`).
+`#frameSkin` inside it is the cosmetic layer, and the two video decks sit inside
+that. New effects should nest a fresh wrapper rather than sharing a transform
+with the bounce loop.