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/public/assets/sounds/README.md

@@ -0,0 +1,9 @@
+Optional audio files. Nothing in here is required: every sound effect and the default music are
+synthesized in the browser (src/audio/synthSounds.ts and src/audio/songs.ts).
+
+- Sound effects: put e.g. `line_clear.wav` here and register it in `soundFiles` in
+  `src/theme/assets.ts` under its SoundId (`lineClear: asset('assets/sounds/line_clear.wav')`).
+- Music: put tracks in `music/` and add them to `musicTracks` in `src/theme/assets.ts`
+  (`{ id: 'my_track', title: 'My Track', kind: 'file', src: asset('assets/sounds/music/my_track.mp3') }`).
+
+Any format the browser can decode works (wav, mp3, ogg). Only use files you have the rights to.