chore: scaffold Vite + Svelte 5 + TS project with chess.js and vitest

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.svelte

@@ -0,0 +1,4 @@
+<main>
+  <h1>Duplicate Chess</h1>
+  <p>Sandbox under construction.</p>
+</main>

src/main.ts

@@ -0,0 +1,9 @@
+import { mount } from 'svelte'
+import './app.css'
+import App from './App.svelte'
+
+const app = mount(App, {
+  target: document.getElementById('app')!,
+})
+
+export default app