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/pages/GamePage.module.css

@@ -1,6 +1,6 @@
 .page {
   min-height: 100vh;
-  padding: 12px 16px 24px;
+  padding: 16px 16px 24px;
 }
 
 .header {
@@ -8,7 +8,23 @@
   align-items: center;
   justify-content: space-between;
   gap: 16px;
+  min-height: 26px;
   margin-bottom: 14px;
+  /* Keep clear of the fixed sound menu in the corner (CornerControls), also while it is open. */
+  padding-right: calc(var(--sound-menu-width, 26px) + 18px);
+}
+
+.headerInfo {
+  display: flex;
+  flex-wrap: nowrap;
+  align-items: center;
+  justify-content: flex-end;
+  gap: 16px;
+}
+
+.track {
+  color: var(--color-muted);
+  font-size: 8px;
 }
 
 .lobbyCode {