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/CornerControls.tsx

@@ -0,0 +1,47 @@
+import { useEffect, useState, type MouseEvent } from 'react';
+import { SoundMenu } from './SoundMenu';
+import { Button } from './ui/Button';
+import styles from './CornerControls.module.css';
+
+const CONFIRM_MS = 3000;
+
+interface Props {
+  /** Shows a Leave button below the sound button. */
+  onLeave?: () => void;
+  /** Ask for a second click before leaving, e.g. because leaving forfeits a running match. */
+  leaveConfirm?: boolean;
+}
+
+/** Sound menu and optional Leave button, fixed in the upper right corner on every screen. */
+export function CornerControls({ onLeave, leaveConfirm = false }: Props) {
+  const [confirming, setConfirming] = useState(false);
+
+  useEffect(() => {
+    if (!confirming) return;
+    const timer = window.setTimeout(() => setConfirming(false), CONFIRM_MS);
+    return () => window.clearTimeout(timer);
+  }, [confirming]);
+
+  const onLeaveClick = (e: MouseEvent<HTMLButtonElement>) => {
+    // Keep keyboard focus off the button so Space (hard drop) doesn't press it.
+    e.currentTarget.blur();
+    if (leaveConfirm && !confirming) setConfirming(true);
+    else onLeave?.();
+  };
+
+  return (
+    <div className={styles.corner}>
+      <SoundMenu />
+      {onLeave && (
+        <Button
+          size="small"
+          variant={confirming ? 'danger' : 'secondary'}
+          onClick={onLeaveClick}
+          title={leaveConfirm ? 'Leaving gives up the running match' : 'Leave the lobby'}
+        >
+          {confirming ? 'Sure?' : 'Leave'}
+        </Button>
+      )}
+    </div>
+  );
+}