fix(ui): own back chevron in Telegram on all platforms; drop the native BackButton

The Telegram native BackButton does not render in the windowed Mini App (the
owner's emulator + a fresh beta TG report backVisible=false on both iOS and
Android), so relying on it lost back navigation — iOS had no back affordance at
all. Show the app's own back chevron whenever there is a back target, on every
platform (Header showBack = !!back), and drop the now-dead native BackButton
effect (App.svelte). The native close control stays — a windowed Mini App
cannot hide it (no Telegram API).

WIP: the temp lobby diagnostic overlay and the requestFullscreen no-op remain
for the owner's emulator test; finalize after confirmation.

ui/src/App.svelte

@@ -2,9 +2,9 @@
   import { onMount } from 'svelte';
   import { cubicOut } from 'svelte/easing';
   import { app, bootstrap } from './lib/app.svelte';
-  import { navigate, router, type RouteName } from './lib/router.svelte';
+  import { router, type RouteName } from './lib/router.svelte';
   import { t } from './lib/i18n/index.svelte';
-  import { insideTelegram, telegramBackButton, telegramChromeDiag } from './lib/telegram';
+  import { insideTelegram, telegramChromeDiag } from './lib/telegram';
   import Toast from './components/Toast.svelte';
   import Splash from './components/Splash.svelte';
   import StaleInviteModal from './components/StaleInviteModal.svelte';
@@ -30,19 +30,6 @@
   // another screen is not covered.
   const routeIsLobby = $derived(router.route.name === 'lobby');
 
-  // Inside Telegram, drive its native header back button: show it on any sub-screen
-  // (everything returns to the lobby root), hide it on the lobby/login. The app's own
-  // back chevron is hidden in Telegram (Header.svelte) so only the native one shows.
-  $effect(() => {
-    if (!insideTelegram()) return;
-    const r = router.route;
-    // The chat / check sub-screens step back to their game; every other sub-screen to the lobby.
-    let target = '/';
-    if (r.name === 'gameChat' || r.name === 'gameCheck') target = `/game/${r.params.id}`;
-    else if (r.name === 'feedback') target = '/about'; // back to the Settings → Info tab
-    telegramBackButton(r.name !== 'lobby' && r.name !== 'login', () => navigate(target));
-  });
-
   // Screen transitions: the lobby is the navigation root. Entering a screen from the
   // lobby slides it in from the right (forward); returning to the lobby slides the
   // screen out to the right and reveals the lobby (back). Transitions are local, so

ui/src/components/Header.svelte

@@ -1,6 +1,5 @@
 <script lang="ts">
   import { navigate } from '../lib/router.svelte';
-  import { insideTelegram, isTelegramAndroid } from '../lib/telegram';
   import { connection } from '../lib/connection.svelte';
   import { t } from '../lib/i18n/index.svelte';
   import { app } from '../lib/app.svelte';
@@ -9,10 +8,10 @@
 
   let { title, back, grow = false }: { title: string; back?: string; grow?: boolean } = $props();
 
-  // Inside Telegram the native header back button (App.svelte) is the back control, so the app's
+  // The app always shows its own back chevron when there is a back target — on every platform, in
-  // own chevron is hidden to avoid two back affordances — except on Android, where the native
+  // and out of Telegram. The native Telegram BackButton is not used: it does not render reliably in
-  // BackButton does not render in the (non-fullscreen) presentation, so the app shows its own.
+  // the windowed Mini App (relying on it would lose back navigation there).
-  const showBack = $derived(!!back && (!insideTelegram() || isTelegramAndroid()));
+  const showBack = $derived(!!back);
 </script>
 
 <header class="nav" class:grow>