Add Svelte GUI, FIT encoder and README

Standalone binary embeds the frontend, avoiding the dev-server dependency
that made the window fail to load.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/src/main.ts

@@ -0,0 +1,8 @@
+import { mount } from 'svelte';
+import './app.css';
+import App from './App.svelte';
+
+const target = document.getElementById('app');
+if (!target) throw new Error('#app mount point missing from index.html');
+
+export default mount(App, { target });