fix(export): VK Android viewer+clipboard; no share title on iOS

Round-5 on-device findings:

- VK Android's DownloadFile hangs regardless of Content-Length and
  Range/206 (PNG stalls ~80%, GCG at 0%) — the download route is
  abandoned there: the PNG opens in VK's native image viewer
  (VKWebAppShowImages — its save works on-device) and the GCG returns
  to the clipboard copy (the pre-URL route, always solid). VK iOS keeps
  VKWebAppDownloadFile (verified perfect); the desktop iframe keeps
  plain anchor downloads.
- The iOS share sheet no longer carries a title: iOS pasted the
  'game-<hash>.png' filename as accompanying text next to the image.

docs/ARCHITECTURE.md

@@ -798,9 +798,11 @@ service ever needs to know the public host) and hands it to the best affordance
 each platform has: Telegram Android/desktop `downloadFile` (Bot API 8.0; the
 chooser there is the native showPopup — activation-free bridge calls end to end),
 Telegram iOS the OS share sheet with the fetched file (the app-modal click supplies
-the user activation a popup callback lacks), VK mobile `VKWebAppDownloadFile` for
-both formats, the OS share sheet on a mobile browser, or a plain anchor download
-(desktop browsers and the VK desktop iframe). The gateway serves the bytes via
+the user activation a popup callback lacks), VK iOS `VKWebAppDownloadFile` for both
+formats, VK Android the native image viewer (PNG) + the clipboard (GCG) — its
+DownloadFile hangs regardless of Content-Length/Range — the OS share sheet on a
+mobile browser, or a plain anchor download (desktop browsers and the VK desktop
+iframe). The gateway serves the bytes via
 `http.ServeContent` (Content-Length + Range/206 — Android's system DownloadManager
 hangs on chunked bodies of unknown length). The GET is the gateway's
 **only unauthenticated data route** (`/dl/*` — in the caddy `@gateway` matcher and the