feat: style the UI with Oat and make the empty canvas a click-to-load drop zone

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/main.ts

@@ -1,3 +1,4 @@
+import '@knadh/oat/oat.min.css';
 import { mount } from 'svelte';
 import App from './App.svelte';