feature: integrate per-stem volume sliders into the mixer popover

[byrongamatos]

Restored from slopsmith/slopsmith#95 — original issue, opened by @byrongamatos on 2026-04-26.
[restored-from: slopsmith/slopsmith#95]

Follow-up from #378 / #94.

slopsmith#94 introduces a fader registry (`window.slopsmith.audio.registerFader`). For sloppak songs with separated stems, each stem (guitar, bass, drums, vocals, piano, other) already has its own GainNode in `window._stemsState`, exposed today via the existing stem panel. Folding them into the mixer popover would give the user one surface for all audio balance: Song · Amp · Guitar Stem · Bass Stem · ...

Considerations

• Stems are per-song state — `_stemsState` is rebuilt on each `playSong()`. Faders would need to register on song load, unregister on song teardown.
• Variable count — depends on what stems were split. Some songs have 2, some have 6.
• Existing stem panel (slopsmith-plugin-stems) overlaps with this functionality. The cleanest path is probably the Stems plugin opting in: register-and-unregister faders on song load/teardown, leaving the existing panel as the more detailed view.
• Mixer popover width — N+2 strips (Song, Amp, plus stems) may overflow the popover. May need horizontal scroll or a configurable hide-by-default mode.

Out of scope for this issue

• Mute/solo per stem (currently in stem panel; could come later).
• Per-arrangement stem presets.

Acceptance criteria

• Sloppak song with 4+ stems → mixer popover shows Song, Amp (if enabled), and one strip per stem.
• Switching songs → stem strips disappear/reappear cleanly without dangling faders.
• Existing stem panel continues to work; no double-control fight.

Reference: parent feature #378, registry implemented in #94.