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.

README.md

@@ -17,6 +17,7 @@ Online player-vs-player falling-block game with items.
 - **1-9** selects the item target (1 = you, 2.. = opponents in the order shown). **F** uses the first item on the target.
 - You never see who targets you, but every opponent's items are shown beneath their mini field.
 - Item names flash on top of the field they hit.
+- Retro sound effects and a background music playlist, all synthesized in the browser. Any sound can be swapped for an audio file. The speaker button in the upper right corner opens volume sliders and mute buttons for effects and music. **M** mutes everything, **B** toggles the music, **N** skips the track.
 
 Controls: arrows or WASD to move/rotate/soft drop, Space for hard drop, Z/Q/Ctrl to rotate left, C/Shift to hold.
 
@@ -74,3 +75,4 @@ The reverse proxy (attached to both networks) should route:
 - Block colors are in `src/theme/colors.ts`.
 - Block drawing style is in `src/render/draw.ts`.
 - Key bindings are in `src/config/keybindings.ts`.
+- Sound files and music tracks are registered in `src/theme/assets.ts` (`soundFiles`, `musicTracks`). All volume levels are in `src/config/sound.ts`.