fix(export): per-platform polish from the on-device review

- TG iOS: the OS share sheet with the fetched file (WKWebView has
  navigator.share — the долгоживущий debug-overlay Share path proves
  it). The chooser stays the app modal on this one platform: a native
  popup callback cannot supply the user activation the sheet needs.
  TG Android/desktop keep the native popup + downloadFile (verified
  perfect, untouched).
- VK mobile: the PNG now rides VKWebAppDownloadFile like the GCG — the
  owner-verified path that lands in VK's native share flow; the
  ShowImages viewer is dropped (its preview offered save only, no
  share).
- VK desktop iframe: both formats are plain anchor downloads (it is an
  ordinary browser; the viewer/bridge path made no sense there).

ui/src/lib/telegram.ts

@@ -387,6 +387,12 @@ export function telegramCanDownloadFile(): boolean {
   return !!webApp()?.downloadFile;
 }
 
+/** telegramPlatform returns the SDK's platform tag ('ios', 'android', 'tdesktop', …) or ''
+ *  outside Telegram — the export delivery branches on it (iOS gets the OS share sheet). */
+export function telegramPlatform(): string {
+  return webApp()?.platform ?? '';
+}
+
 /**
  * telegramDownloadFile asks Telegram to download url as fileName through its native
  * dialog — the Mini App file delivery that works on every Telegram platform (a webview