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/e2e/landing.spec.ts

@@ -19,3 +19,17 @@ test('landing shows the pitch, switches language via the dropdown, and toggles t
   const after = await page.evaluate(() => document.documentElement.getAttribute('data-theme'));
   expect(after).not.toBe(before);
 });
+
+// The document-pin that stops the SPA from rubber-banding is scoped to the game app (main.ts
+// adds .app-shell); the landing is a normal scrolling document and must keep scrolling.
+test('the landing is a normal scrolling document (the SPA document-pin does not apply)', async ({ page }) => {
+  await page.goto('/landing.html');
+  await expect(page.getByText(/Play Scrabble/i)).toBeVisible();
+
+  const state = await page.evaluate(() => ({
+    shell: document.documentElement.classList.contains('app-shell'),
+    bodyPosition: getComputedStyle(document.body).position,
+  }));
+  expect(state.shell).toBe(false);
+  expect(state.bodyPosition).not.toBe('fixed');
+});