UI: pin the SPA document so iOS/WKWebView cannot rubber-band the page

On iOS (notably the Telegram Mini App) the document elastic-overscrolls on a
vertical drag even with overscroll-behavior:none — the whole page stretches and
bounces, and it fought the board's swipe-to-open-history. Telegram's own
swipe-to-minimise is already disabled at launch; this removes the remaining
WebKit document bounce by pinning the document (position:fixed + overflow:hidden)
for the game SPA only. Every screen already fits the visual viewport (--vvh) and
scrolls its own inner areas, so the document never needed to scroll.

Scoped to the app via an `app-shell` class set in main.ts; the standalone
landing page (landing.ts) keeps its normal scrolling document. e2e locks the
contract on both entries.

ui/src/main.ts

@@ -2,4 +2,9 @@ import { mount } from 'svelte';
 import './app.css';
 import App from './App.svelte';
 
+// Pin the document for the game SPA (see app.css `html.app-shell`) so iOS/WKWebView — notably
+// the Telegram Mini App — cannot rubber-band the whole page on a vertical drag. The standalone
+// landing page (landing.ts) is a normal scrolling document and deliberately omits this class.
+document.documentElement.classList.add('app-shell');
+
 export default mount(App, { target: document.getElementById('app')! });