Migrate app to SvelteKit, add components, static build config and GH Pages deploy workflow

[antiero]

Motivation

• Standardise the project on Svelte/SvelteKit and remove the legacy React entrypoints to simplify the runtime and eliminate duplicate implementations.
• Prepare the app for static hosting (GitHub Pages) by adding static adapter settings, base path handling, and a deploy workflow.
• Provide documentation and a clearer architecture surface for maintainability and future contributors.

Description

• Replaced the React app with a SvelteKit single-page app by removing src/App.tsx and src/main.tsx and converting the UI into src/routes/+page.svelte plus new components src/lib/components/ControlPanel.svelte, TouchSurface.svelte, and PolyfieldHeader.svelte to modularise the UI.
• Updated build and dev toolchain to SvelteKit by changing vite.config.ts to use the sveltekit plugin, adding svelte.config.js configured with @sveltejs/adapter-static (output to dist with 404.html fallback and trailingSlash: 'always'), and adjusted tsconfig.json/package.json (dependencies/devDependencies and scripts) for Svelte and Svelte tooling.
• Added GitHub Pages deployment support with .github/workflows/deploy.yml that auto-computes BASE_PATH for project vs user pages, builds the app, uploads the dist artifact, and deploys via actions/deploy-pages; added static/.nojekyll and .gitignore entry for .svelte-kit.
• Small code and typing updates including src/audio/engine.ts import typing change and refactors in +page.svelte to wire state to the new components, plus a new ARCHITECTURE.md describing runtime, event flow, and module responsibilities.

Testing

• No automated tests were executed as part of this change; CI will run the included GitHub Actions workflow on pushes to main to build and deploy the static site.

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Codex Task