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/lib/telegram.test.ts

@@ -7,7 +7,6 @@ import {
   routeExternalLinkInTelegram,
   telegramLaunch,
   telegramOpenExternalLink,
-  telegramRequestFullscreen,
 } from './telegram';
 
 function stubWebApp(initData: string, startParam?: string) {
@@ -47,44 +46,6 @@ describe('telegram launch detection', () => {
   });
 });
 
-// stubClient stands up a fake WebApp on the given platform with a requestFullscreen spy, so the
-// platform gate can be asserted without a real Telegram client.
-function stubClient(platform?: string) {
-  const requestFullscreen = vi.fn();
-  vi.stubGlobal('window', { Telegram: { WebApp: { platform, requestFullscreen } } });
-  return { requestFullscreen };
-}
-
-const desktopPlatforms = ['tdesktop', 'macos', 'web', undefined];
-
-describe('telegramRequestFullscreen', () => {
-  afterEach(() => vi.unstubAllGlobals());
-
-  // TEMP: fullscreen is disabled on all platforms for an owner test (telegramRequestFullscreen is a
-  // no-op); restore this (toHaveBeenCalledOnce) and un-skip when the iOS path is restored.
-  it.skip('goes immersive fullscreen on iOS', () => {
-    const { requestFullscreen } = stubClient('ios');
-    telegramRequestFullscreen();
-    expect(requestFullscreen).toHaveBeenCalledOnce();
-  });
-
-  it('stays windowed on Android, where fullscreen hides the native back button', () => {
-    for (const p of ['android', 'android_x']) {
-      const { requestFullscreen } = stubClient(p);
-      telegramRequestFullscreen();
-      expect(requestFullscreen, `platform=${p}`).not.toHaveBeenCalled();
-    }
-  });
-
-  it('leaves desktop clients as a standard window', () => {
-    for (const p of desktopPlatforms) {
-      const { requestFullscreen } = stubClient(p);
-      telegramRequestFullscreen();
-      expect(requestFullscreen, `platform=${p}`).not.toHaveBeenCalled();
-    }
-  });
-});
-
 describe('telegramOpenExternalLink', () => {
   afterEach(() => vi.unstubAllGlobals());