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();
+});

ui/src/App.svelte

@@ -141,8 +141,6 @@
     position: relative;
     height: 100%;
     overflow: hidden;
-    /* Keep a horizontal drag from also triggering the browser's history-swipe underneath. */
-    overscroll-behavior-x: contain;
   }
   .pane {
     position: absolute;

ui/src/lib/backswipe.svelte.ts

@@ -9,7 +9,6 @@
 import { app } from './app.svelte';
 import { createBackSwipe, shouldArm } from './backswipe';
 import { navigate } from './router.svelte';
-import { insideTelegram } from './telegram';
 
 /**
  * backSwipe is the reactive view of the in-flight gesture. App binds `progress` to the
@@ -127,7 +126,6 @@ export function initBackSwipe(getParent: () => string | null): () => void {
         width: window.innerWidth,
         pointerType: e.pointerType,
         hasParent: !!parent,
-        insideTelegram: insideTelegram(),
       })
     )
       return;

ui/src/lib/backswipe.test.ts

@@ -7,7 +7,6 @@ describe('shouldArm', () => {
     width: 400,
     pointerType: 'touch',
     hasParent: true,
-    insideTelegram: false,
   };
 
   it('arms a touch press inside the left band of a screen with a parent', () => {
@@ -15,11 +14,10 @@ describe('shouldArm', () => {
     expect(shouldArm({ ...base, clientX: 0.38 * 400 - 1 })).toBe(true);
   });
 
-  it('does not arm past the band, for a mouse, at a root, or inside Telegram', () => {
+  it('does not arm past the band, for a mouse, or at a root', () => {
     expect(shouldArm({ ...base, clientX: 0.38 * 400 + 1 })).toBe(false);
     expect(shouldArm({ ...base, pointerType: 'mouse' })).toBe(false);
     expect(shouldArm({ ...base, hasParent: false })).toBe(false);
-    expect(shouldArm({ ...base, insideTelegram: true })).toBe(false);
   });
 
   it('honours a custom band fraction', () => {

ui/src/lib/backswipe.ts

@@ -30,18 +30,18 @@ export interface ArmInput {
   pointerType: string;
   /** Whether the current screen has a parent to return to (false on a navigation root). */
   hasParent: boolean;
-  /** Whether the app is running inside Telegram, where its native Back button owns this. */
-  insideTelegram: boolean;
   /** Fraction of the width forming the left activation band; defaults to 0.38. */
   bandFrac?: number;
 }
 
 /**
  * shouldArm decides whether a pointerdown may begin a back-swipe: a touch or pen press in
- * the left band of a screen that has a parent, while not inside Telegram.
+ * the left band of a screen that has a parent. It is intentionally Telegram-agnostic —
+ * Telegram's only close/minimise gesture is vertical (handled by disableVerticalSwipes), so
+ * a horizontal back-swipe has nothing to conflict with there.
  */
 export function shouldArm(i: ArmInput): boolean {
-  if (!i.hasParent || i.insideTelegram) return false;
+  if (!i.hasParent) return false;
   if (i.pointerType === 'mouse') return false;
   const band = (i.bandFrac ?? 0.38) * i.width;
   return i.clientX <= band;