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.

doc/game-rules.md

@@ -159,5 +159,9 @@ Bindings use physical key positions (`KeyboardEvent.code`), so they work on any
 | Hold | C, Shift |
 | Select item target | 1–9 (1 = yourself) |
 | Use item | F |
+| All sound on / off | M |
+| Background music on / off (effects stay on) | B |
+| Volume and mute for effects and music | Speaker button, upper right corner |
+| Next music track | N |
 
 Holding left/right repeats the move after 167 ms, then every 33 ms (DAS/ARR). When both directions are held, the most recently pressed one wins.