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/FUNCTIONAL.md

@@ -345,9 +345,11 @@ stretches the board (never below its minimum) so the image carries no dead space
 Delivery is **one signed, short-lived link for both formats on every platform**,
 handed to the best affordance each platform has (each branch verified on-device):
 Telegram Android/desktop use Telegram's download dialog (whose own preview shares
-onwards); **Telegram iOS opens the OS share sheet** with the fetched file; on VK
-both formats go through VK's download into its native share/preview flow (the
-desktop VK iframe, an ordinary browser, downloads them); a **mobile browser gets
+onwards); **Telegram iOS opens the OS share sheet** with the fetched file; VK iOS
+takes both formats through VK's download into its native share flow, while on the
+**VK Android client** — whose downloader hangs — the image opens in VK's native
+photo viewer (saving from its controls) and the GCG copies to the clipboard (the
+desktop VK iframe, an ordinary browser, downloads both); a **mobile browser gets
 the OS share sheet** (nothing lands in Downloads first); a desktop browser
 downloads the file. The link needs no login to fetch (the platforms' downloaders
 carry none) and is valid for minutes. The single exception is a legacy Telegram