audio: make ambient music polyphonic with real songs, fix engine silence

Rewrite all 19 theme ambient presets with chord progressions, bass drones,
overlapping chord pads (dur > step on melodyNote), and swing melodies.
Add per-theme 16-step drum patterns (kick, snare, hat, clap) synthesized
in the Web Audio engine.

Go (theme_sounds.go):
- Add Step field to melodyNote for sequencer timing independent of duration
- Add Drums []string field to ambientPreset for 16-step patterns
- Add buildMeasure, buildSong, buildSwingMelody, and pat helpers
- Rewrite every sounds*() func with I–IV–V–I progressions, bass, and melody

JS (shared.js):
- Fix startEngine() to await ctx.resume() before scheduling
- Make masterGain a binary gate (fade to 1.0 in ~50 ms), not preset.gain
- Fix startDrones() to scale by preset.gain instead of 1.0/numDrones
- Add twin-oscillator detuned timbre in playMelodyNote()
- Add synthesized drum engine: playDrum(), scheduleDrums(), stopDrums()
- Keep splash sounds bypassing masterGain so they never get gated out

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