UI: enable the back-swipe inside Telegram (fix lost swipe-back)

The gesture was disabled inside Telegram on the mistaken assumption it would
fight a Telegram horizontal close gesture — but Telegram's only close/minimise
gesture is vertical (disableVerticalSwipes, Bot API 7.7+). Disabling it there,
together with dropping the old left-edge swipe, removed every swipe-back in
Telegram (which that edge-swipe — deliberately enabled in TG, commit 7e34897 —
had provided).

Make shouldArm Telegram-agnostic (drop the insideTelegram guard + its now-unused
import); the gesture works in Telegram too, alongside the native BackButton.
Drop the redundant overscroll-behavior-x on .router (body already sets
overscroll-behavior: none). Regression: an e2e drives the swipe under an
injected Telegram WebApp stub and asserts it navigates back.

ui/e2e/backswipe.spec.ts

@@ -22,6 +22,22 @@ async function openAnnGame(page: Page): Promise<void> {
   await page.waitForTimeout(400); // let the open slide settle before measuring/aiming
 }
 
+// A minimal Telegram WebApp stub: non-empty initData makes insideTelegram() true and drives
+// the Mini App auto-auth into the lobby (the mock gateway accepts any initData).
+function telegramStub() {
+  return {
+    Telegram: {
+      WebApp: {
+        initData: 'query_id=test&user=%7B%22id%22%3A1%7D&auth_date=1&hash=deadbeef',
+        initDataUnsafe: {},
+        themeParams: {},
+        ready() {},
+        expand() {},
+      },
+    },
+  };
+}
+
 /** touchDrag dispatches a single-finger touch drag from (x0,y0) to (x1,y1) over the window. */
 async function touchDrag(page: Page, x0: number, y0: number, x1: number, y1: number): Promise<void> {
   await page.evaluate(
@@ -88,3 +104,20 @@ test('a vertical drag in the band scrolls instead of navigating', async ({ page
   await page.waitForTimeout(300);
   expect(page.url()).toContain('/game/');
 });
+
+test('the back-swipe also works inside Telegram (its only swipe gesture is vertical)', async ({ page }) => {
+  await page.addInitScript((stub) => {
+    Object.assign(window, stub);
+  }, telegramStub());
+  await page.goto('/');
+
+  // Telegram auto-authenticates into the lobby (no guest click), then open the seeded game.
+  await page.getByRole('button', { name: /Ann/ }).click();
+  await expect(page.locator('[data-cell]').first()).toBeVisible();
+  await page.waitForTimeout(400);
+
+  await touchDrag(page, START_X, MID_Y, 300, MID_Y);
+
+  await expect(page).toHaveURL(/#\/$/);
+  await expect(page.getByRole('button', { name: /Ann/ })).toBeVisible();
+});