feature: piano + drums plugins should register synth volume faders

[byrongamatos]

Restored from slopsmith/slopsmith#96 — original issue, opened by @byrongamatos on 2026-04-26.
[restored-from: slopsmith/slopsmith#96]

Follow-up from #378 / #94.

slopsmith#94 introduces a fader registry (`window.slopsmith.audio.registerFader`). The piano and drums visualization plugins each have their own `synthVolume` slider in their config screens, persisted to plugin-prefixed localStorage keys (`piano_synth_vol`, `drums_synth_vol`). They could each register a mixer fader so users can balance piano/drum synth volume from the player popover alongside Song and Amp.

Each migration is small (≤15 lines per plugin) — looks like the NAM PR (slopsmith-plugin-nam-tone#1):

```js
function _registerFader() {
const api = window.slopsmith && window.slopsmith.audio;
if (!api) return;
api.registerFader({
id: 'piano_synth',
label: 'Piano',
min: 0, max: 1, step: 0.05,
defaultValue: _cfg.synthVolume,
getValue: () => _cfg.synthVolume,
setValue: (v) => _setSynthVolume(v), // existing setter — writes _synthGain.gain.value, persists, updates label
});
}
```

Plugins to update

• slopsmith-plugin-piano — register `{id: 'piano_synth', label: 'Piano'}` fader
• slopsmith-plugin-drums — register `{id: 'drums_synth', label: 'Drums'}` fader

Acceptance criteria

• Piano plugin loaded → mixer popover shows Piano fader; existing in-screen slider stays in sync (both call same setter).
• Drums plugin loaded → mixer popover shows Drums fader.
• All three (piano + drums + amp) loaded with sloppak song → popover shows Song, Amp (NAM), Piano, Drums; each fader independently controls its source.

Reference: parent feature #378, registry implemented in #94, NAM as canonical reference: byrongamatos/slopsmith-plugin-nam-tone#1.