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

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

docs/FUNCTIONAL_ru.md

@@ -355,10 +355,12 @@ Telegram Android/desktop (там это безопасно: цепочка це
 платформе**, отданная лучшему механизму, который у платформы реально есть (каждая
 ветка проверена на устройстве): Telegram Android/desktop — диалог загрузки Telegram
 (из его превью файл шерится дальше); **Telegram iOS — системная share-шторка** со
-скачанным файлом; в VK оба формата идут через загрузку VK в её нативный
-share/preview-поток (десктопный iframe VK — обычный браузер — скачивает файлы);
-**мобильный браузер получает системную share-шторку** (ничего не оседает в
-Загрузках); десктопный браузер скачивает файл. Ссылка не требует логина при
+скачанным файлом; VK iOS ведёт оба формата через загрузку VK в её нативный
+share-поток, а на **VK Android** — чей загрузчик виснет — картинка открывается в
+нативном просмотрщике фото VK (сохранение его кнопками), GCG копируется в буфер
+(десктопный iframe VK — обычный браузер — скачивает оба); **мобильный браузер
+получает системную share-шторку** (ничего не оседает в Загрузках); десктопный
+браузер скачивает файл. Ссылка не требует логина при
 скачивании (родные загрузчики платформ его не несут) и живёт минуты. Единственное
 исключение — устаревший клиент Telegram без диалога загрузки (до Bot API 8.0): там
 GCG откатывается на прежнее копирование в буфер (с подтверждающим тостом), а пункт

docs/UI_DESIGN.md

@@ -414,10 +414,12 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
   both options need the connection). Both formats mint the same signed relative link
   (`game.export_url`), resolved against the app's own origin, then delivered per platform
   (each branch owner-verified on-device): TG Android/desktop `downloadFile` (its preview
-  shares onwards); TG iOS the OS share sheet with the fetched file; VK mobile
-  `VKWebAppDownloadFile` for both formats (VK's native share/preview flow), the VK desktop
-  iframe plain anchor downloads; a mobile browser the OS share sheet (the proven
-  fetch-then-share pattern); a desktop browser an anchor download. A legacy Telegram
+  shares onwards); TG iOS the OS share sheet with the fetched file; VK iOS
+  `VKWebAppDownloadFile` for both formats (VK's native share flow); VK Android — whose
+  DownloadFile hangs — the PNG in VK's native image viewer (`VKWebAppShowImages`, its save
+  works) and the GCG to the clipboard; the VK desktop iframe plain anchor downloads; a
+  mobile browser the OS share sheet (the proven fetch-then-share pattern); a desktop
+  browser an anchor download. A legacy Telegram
   client without `downloadFile` keeps the old GCG clipboard copy, hides the image option
   and stays on the app modal. Confirming a resign reveals the full board: it closes the
   history drawer (portrait) and zooms the board out.