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/App.svelte

@@ -4,7 +4,7 @@
   import { app, bootstrap } from './lib/app.svelte';
   import { navigate, router, type RouteName } from './lib/router.svelte';
   import { t } from './lib/i18n/index.svelte';
-  import { insideTelegram, telegramBackButton } from './lib/telegram';
+  import { insideTelegram, telegramBackButton, telegramChromeDiag } from './lib/telegram';
   import Toast from './components/Toast.svelte';
   import Splash from './components/Splash.svelte';
   import StaleInviteModal from './components/StaleInviteModal.svelte';
@@ -138,6 +138,14 @@
   <Splash />
 {/if}
 
+<!-- TEMP DIAGNOSTIC overlay (Android nav debug) — REMOVE before merge -->
+{#if app.ready && insideTelegram()}
+  {#key router.route.name + (router.route.params.id ?? '')}
+    <pre
+      style="position:fixed;left:0;right:0;bottom:0;z-index:9999;margin:0;padding:6px 8px;font:10px/1.35 ui-monospace,monospace;white-space:pre-wrap;overflow-wrap:anywhere;background:rgba(0,0,0,0.85);color:#3f3;max-height:45vh;overflow:auto;pointer-events:none">{telegramChromeDiag()}</pre>
+  {/key}
+{/if}
+
 <style>
   .splash {
     height: 100%;