Add sound effects, background music and a sound menu

Generate retro sound effects and music in the browser with the Web Audio
API, so no audio files are needed. Any sound can still be replaced by a
file.

Audio (src/audio/):
- 16 effects (move, rotate, hold, hard drop, lock, line clears, garbage,
  items, countdown, knockout, win/lose), triggered by comparing match
  snapshots, so the server is unchanged
- Background music as a shuffled playlist of any number of tracks,
  either synth songs or audio files; 3 original songs included, tempo
  rises with the level
- SoundPlayer interface, so the audio engine can be replaced
- Audio files and music tracks are registered in theme/assets.ts
  (soundFiles, musicTracks)
- All volume levels for developers are in config/sound.ts (master, sfx,
  music, per effect, per track, default slider positions)

UI:
- Speaker button fixed in the top right corner of every screen; opens an
  inline strip with a mute button and volume slider for SFX and music
  that doesn't cover the boards or pause the game
- Leave button below it in the lobby and the game; in a running match
  it asks for a second click because leaving gives up the match
- Lobby Music panel to switch single tracks on or off
- Keys: M all sound on/off, B music on/off, N next track
- Settings are saved in localStorage

Docs: audio section in frontend.md, sound and music guide in
extending.md, new keys in game-rules.md and the README.

actionBlocks_website/src/components/MusicTracks.tsx

@@ -0,0 +1,30 @@
+import { useSound } from '../audio/SoundContext';
+import { musicTracks } from '../theme/assets';
+import styles from './MusicTracks.module.css';
+
+/** One checkbox per registered music track; unchecked tracks are skipped by the playlist. */
+export function MusicTracks() {
+  const { settings, setSettings } = useSound();
+
+  const toggleTrack = (id: string, enabled: boolean) =>
+    setSettings({
+      disabledTracks: enabled ? settings.disabledTracks.filter((t) => t !== id) : [...settings.disabledTracks, id],
+    });
+
+  return (
+    <ul className={styles.tracks}>
+      {musicTracks.map((track) => (
+        <li key={track.id}>
+          <label>
+            <input
+              type="checkbox"
+              checked={!settings.disabledTracks.includes(track.id)}
+              onChange={(e) => toggleTrack(track.id, e.target.checked)}
+            />
+            {track.title}
+          </label>
+        </li>
+      ))}
+    </ul>
+  );
+}