Add SCSS and TypeScript build pipeline (esbuild + Dart Sass)

Mirrors the agentception frontend stack: - package.json: esbuild (JS/TS), sass (SCSS), typescript (type-check) - tsconfig.json: strict browser-target config, noEmit, bundler resolution - SCSS: split CSS into partials (_tokens, _components, _layout, _icons, _music) compiled via Dart Sass into static/app.css; embed.css stays standalone - TypeScript: musehub.js → musehub.ts, audio-player.js → audio-player.ts, piano-roll.js → piano-roll.ts; app.ts entry bundles all to static/app.js - All public functions explicitly attached to window for inline HTML handlers - Fix stale CSS token names (--color-surface/border → --bg-surface/border-default) - base.html: 5 CSS links → single app.css; musehub.js → app.js - npm run build | watch:js | watch:css | type-check scripts