feat(ui): drop Telegram fullscreen; own back chevron everywhere; hidden debug panel

Finalises the Telegram Mini App navigation work after on-device testing (Pixel
10 / Android 17 + iOS, fresh beta clients):

- Remove requestFullscreen entirely. Immersive fullscreen hid Telegram's native
  header (and its BackButton) and the Android system swipe-back minimised the
  app; the owner prefers the windowed full-size (expand) presentation, so the
  app never requests fullscreen on any platform now.
- The app's own back chevron (Header, showBack = !!back) drives back-navigation
  on every platform; the native Telegram BackButton is dropped — it does not
  render in the windowed Mini App (backVisible=false on iOS and Android), so
  relying on it lost back navigation (notably none on iOS).
- Replace the temporary always-on diagnostic overlay with a hidden debug panel
  (components/DebugPanel): ten quick taps on the header title open it; it shows a
  privacy-safe client diagnostic snapshot (app version, locale, online, userId,
  Telegram chrome / viewport / SDK state — no secrets, no IP) and shares it via
  the OS share sheet / clipboard; a tap anywhere except Share dismisses it.
- Drop the now-dead telegramRequestFullscreen / telegramBackButton /
  isTelegramAndroid helpers and the iOS-fullscreen unit test.

Telegram has no native non-modal notification API (only modal showPopup /
showAlert), so in-app toasts stay ours. Docs: UI_DESIGN.md.

ui/src/App.svelte

@@ -4,11 +4,11 @@
   import { app, bootstrap } from './lib/app.svelte';
   import { router, type RouteName } from './lib/router.svelte';
   import { t } from './lib/i18n/index.svelte';
-  import { insideTelegram, telegramChromeDiag } from './lib/telegram';
   import Toast from './components/Toast.svelte';
   import Splash from './components/Splash.svelte';
   import StaleInviteModal from './components/StaleInviteModal.svelte';
   import WelcomeRedeemModal from './components/WelcomeRedeemModal.svelte';
+  import DebugPanel from './components/DebugPanel.svelte';
   import Login from './screens/Login.svelte';
   import Lobby from './screens/Lobby.svelte';
   import NewGame from './screens/NewGame.svelte';
@@ -125,12 +125,8 @@
   <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 app.debugOpen}
+  <DebugPanel />
 {/if}
 
 <style>