feat: scaffold Vite React app

[britle20]

Summary

Closes #6.

Creates the initial Vite + React + TypeScript app scaffold for the browser-based mini DAW without implementing DAW features.

Changes

• Added npm package scripts for dev, typecheck, lint, test, build, and preview.
• Added Vite, React, TypeScript, ESLint, and Vitest configuration.
• Added initial src layout for app, components, features, audio, model, persistence, utils, and styles.
• Added CSS Modules app shell with transport/editor placeholders and primitive-to-semantic design tokens.
• Updated README status and marked the scaffold feature spec as in review.

Testing

• npm run typecheck
• npm run lint
• npm run test
• npm run build
• npm run dev -- --host 127.0.0.1, then curl -I http://127.0.0.1:5173/ returned 200 OK

Notes / Limitations

• No audio engine, drum sequencer, piano roll, samples, persistence, or arrangement features are implemented in this PR.
• Vitest is configured and currently passes with no test files; feature work should add focused tests as behavior is introduced.