chore(ui): TEMP disable fullscreen for testing + Android back chevron + BackButton diag

WIP for the Android nav investigation (TEMP bits reverted before merge):
- telegramRequestFullscreen: temporarily a no-op (incl. iOS) so the owner can
  confirm the Android "fullscreen look" is Telegram's own Mini App presentation,
  not our requestFullscreen (isFullscreen is already false on Android).
- Header: show the app's own back chevron in Telegram on Android, where the
  native BackButton does not render — a reliable tap-back. [keep]
- Diagnostic overlay moved app-wide (pointer-events:none) and now reports
  BackButton state (req/present/visible) + viewport geometry, to see whether the
  native BackButton can capture the Android system swipe-back.

ui/src/components/Header.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import { navigate } from '../lib/router.svelte';
-  import { insideTelegram } from '../lib/telegram';
+  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';
@@ -9,9 +9,10 @@
 
   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 own chevron is hidden to avoid two back affordances.
-  const showBack = $derived(!!back && !insideTelegram());
+  // Inside Telegram the native header back button (App.svelte) is the back control, so the app's
+  // own chevron is hidden to avoid two back affordances — except on Android, where the native
+  // BackButton does not render in the (non-fullscreen) presentation, so the app shows its own.
+  const showBack = $derived(!!back && (!insideTelegram() || isTelegramAndroid()));
 </script>
 
 <header class="nav" class:grow>