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/smoke.spec.ts

@@ -28,3 +28,20 @@ test('guest reaches a board and previews a placement', async ({ page }) => {
   // The contextual MakeMove control (✅) appears once a tile is pending.
   await expect(page.locator('.make')).toBeVisible();
 });
+
+// The SPA pins the document so iOS/WKWebView cannot rubber-band the whole page on a vertical
+// drag (the elastic bounce that fought the board's swipe-to-open-history). The native bounce
+// itself is not reproducible in Playwright, so we assert the CSS contract that suppresses it.
+test('the app pins the document so the page cannot rubber-band', async ({ page }) => {
+  await page.goto('/');
+  await expect(page.getByRole('button', { name: /guest/i })).toBeVisible();
+
+  const lock = await page.evaluate(() => ({
+    shell: document.documentElement.classList.contains('app-shell'),
+    htmlOverflow: getComputedStyle(document.documentElement).overflowY,
+    bodyPosition: getComputedStyle(document.body).position,
+  }));
+  expect(lock.shell).toBe(true);
+  expect(lock.htmlOverflow).toBe('hidden');
+  expect(lock.bodyPosition).toBe('fixed');
+});