feat: implement CSS Modules main DAW UI

[britle20]

Summary

Closes #11.

Converts the Tailwind reference direction into a CSS Modules-based main DAW UI shell with local UI state for transport, mode, BPM, sidebar selection, and drum step toggles.

Changes

• Added shared Icon and Panel components.
• Added TransportBar, ProjectSidebar, DrumSequencer, and PianoRoll feature components.
• Added piano keyboard and velocity editor visuals with inline geometry only where dynamic editor positioning is needed.
• Reworked global styles and primitive/semantic design tokens for the dark DAW UI.
• Loaded Inter and Material Symbols through index.html.
• Updated UI docs and active plan status for the UI shell task.

Testing

• npm run typecheck
• npm run lint
• npm run test
• npm run build
• npm run dev -- --host 127.0.0.1 --port 5173, then curl -I http://127.0.0.1:5173/

Screenshots

Not captured from this environment.

Notes / Limitations

• Tailwind was not added.
• Audio playback, lookahead scheduling, real piano roll editing, persistence, and arrangement editing are intentionally excluded.
• Existing audio sample playback engine and bundled sample assets were preserved, but the new main shell does not wire them into transport playback yet.