feat(export): platform-native delivery — TG popup+downloadFile, VK viewer, mobile share sheet

Per the on-device review, the last hop is now the most native affordance
per platform, all fed by the same signed URL:

- Telegram: the native showPopup chooser returns (safe here — the whole
  TG chain is bridge calls, which need no user activation) and both
  formats go to the native downloadFile dialog on iOS and Android alike.
- VK: the PNG opens in VK's native photo viewer (VKWebAppShowImages) —
  on screen at once, saved/shared from the viewer's own controls; the
  GCG keeps VKWebAppDownloadFile; either falls back to a plain anchor
  download (the desktop iframe). The gateway now serves /dl/* via
  http.ServeContent (Content-Length + Range/206) — another swing at the
  VK Android DownloadManager hang; if it persists, the next step is the
  clipboard fallback for the VK-Android GCG.
- Mobile browsers: the OS share sheet with the fetched file (the proven
  fetch-then-share pattern) — nothing lands in Downloads first; desktop
  keeps the anchor download.

Legacy Telegram (< 8.0, no downloadFile) keeps the app modal + GCG
clipboard copy and hides the image option.

ui/src/lib/i18n/en.ts

@@ -311,6 +311,7 @@ export const en = {
   'stats.guestHint': 'Sign in to track your statistics.',
 
   'game.exportGcg': 'Export game',
+  'game.exportChoice': 'Export the finished game as:',
   'game.exportImageOpt': 'Image (PNG)',
   'game.exportGcgOpt': 'GCG file',
   'game.gcgActiveOnly': 'Available once the game is finished.',