feat: add arrangement view shell

[britle20]

Summary

Closes #26

Adds the SONG-mode arrangement view UI shell while preserving the existing transport bar, project sidebar, and PAT-mode hybrid clip editor.

Changes

• Added ArrangementView with CSS Modules-based track headers, timeline ruler, beat/lane grid, static clip blocks, and arrangement navigation footer.
• Switched the main workspace between PAT clip editing and SONG arrangement view based on the existing PAT/SONG transport mode.
• Kept the Google Stitch/Tailwind prototype as visual reference only; no Tailwind dependency or utility-class UI was added.
• Updated PLANS.md and feature spec statuses for the completed tempo task and current arrangement task.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present

Additional checks:

• Verified no Tailwind-related matches with rg "tailwind|cdn.tailwind|class=\".*bg-|className=\".*bg-" . -n --glob "!node_modules/**" --glob "!dist/**".
• Started the dev server during implementation; Vite served successfully on http://127.0.0.1:5174/ because 5173 was occupied.

Screenshots

Not included from this environment. Manual browser review is recommended for exact visual alignment.

Notes / Limitations

• This is a static/interactable UI shell only.
• No arrangement data model, clip instance persistence, arrangement playback, drag/drop editing, real waveform rendering, or zoom behavior is implemented yet.
• Demo clips and tracks are local mock data inside the arrangement view component.

Follow-up

• Implement the arrangement data model before real arrangement editing/playback.
• Add playhead integration and scrolling behavior when arrangement playback exists.