feat: add arrangement mixer panel shell

[britle20]

Summary

• Adds a bottom dock mixer panel to the SONG arrangement view.
• Renders track channel strips for the visible arrangement tracks plus a master strip.
• Adds local UI state for volume faders, mute/solo toggles, and effect-slot placeholders.
• Uses mock/static level meter visuals only; no audio routing or analyser data is connected.
• Updates the feature spec status and Active Issue Order.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present
• npm run dev -- --host 127.0.0.1 --port 5173, served on 5174 because 5173 was in use, then curl -I http://127.0.0.1:5174/ returned HTTP 200

Notes / Limitations

• This is a UI shell only. Volume, mute, solo, meters, and FX slots do not affect Web Audio output.
• Level meters are mock visual values, not real signal metering.
• The mixer strip area scrolls horizontally when channels exceed the available width.
• PLANS.md marks Drum step subdivisions #32 as PR feat: add drum step subdivisions #37 in review because Drum step subdivisions #32 is currently represented by a separate open PR.

Closes #34