[Rebrand] Move Music Lab theme boundary to root

[ebeastlake]

Summary

Follow-up to #73649 for Sanchit's review comment about data-theme="Dark" living lower in the tree.

This moves the Music Lab dark design-system theme boundary to the Music Lab root (#music-lab) and removes the now-redundant local boundaries from:

• PackDialog
• Controls
• Timeline
• Blockly workspace div
• SoundsPanel filter controls
• InstrumentGrid

No token, color, layout, or behavior changes intended. This is the small architectural follow-up to Hannah's #73300 note: Music Lab did not have a root DS dark theme yet, so earlier rebrand work added local boundaries where needed.

Testing story

Static checks run:

• ./tools/hooks/pre-commit
• yarn run typecheck from apps/
• rg -n 'data-theme="Dark"|data-theme=\x27Dark\x27' apps/src/music shows only apps/src/music/views/MusicLabView.tsx

Local dashboard was not running on localhost:9000 or localhost:3000 when I made this follow-up, so I could not take a Playwright screenshot from this environment.

Hey, human. Add screenshots here before marking ready:

Before screenshot

After screenshot

Manual pass to do on a running dashboard:

• Compare this branch against [Rebrand] Update Music Lab pack dialog #73649. The Music Lab new-project pack dialog should be visually unchanged.
• Verify Controls, Timeline, Blockly workspace/toolbox, SoundsPanel filters, and InstrumentGrid still resolve dark DS tokens.
• Enable DCDO.set('brand-router-enabled', true) and test ?brand=codeai-next so the pink audit still covers all surfaces from [Rebrand] MusicLab -> Design System #73300 plus PackDialog.
• Recheck the PackDialog states from [Rebrand] Update Music Lab pack dialog #73649: default, selected row, sort modes, hover/focus, keyboard nav, and screen reader announcement.
• Widths: 1568, 1280, 1024, 768. Browsers: Chrome, Firefox, Safari. Also check RTL if time.