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).

docs/UI_DESIGN.md

@@ -406,21 +406,21 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
 - **History / export**: the in-game slide-down history lays each move out in a per-seat grid
   (the word(s) and the move score, no running total); the 📤 in the history header appears
   only once the game is finished — never in an honest-AI game (throwaway practice) — and
-  opens the **format chooser**: Telegram's native popup inside Telegram — safe there,
-  since the whole TG chain is bridge calls needing no user activation (a popup callback
+  opens the **format chooser**: Telegram's native popup on TG Android/desktop — safe
+  there, since that chain is all bridge calls needing no user activation (a popup callback
   must never lead into `navigator.share`/clipboard, the on-device finding) — and the app's
-  own modal elsewhere (VK has no native chooser; the accent button leads with the image,
+  own modal elsewhere (TG iOS delivers via the OS share sheet, which needs the modal
+  click's activation; VK has no native chooser; the accent button leads with the image,
   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 natively:
-  Telegram `downloadFile` (both TG platforms); on VK the PNG opens in the native photo
-  viewer (`VKWebAppShowImages`, save/share from its controls) and the GCG goes through
-  `VKWebAppDownloadFile`, both falling back to a plain anchor download (the desktop
-  iframe); a mobile browser fetches the bytes and opens the OS share sheet
-  (`navigator.share` with the file — the proven fetch-then-share pattern), a desktop
-  browser downloads by anchor. 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.
+  (`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
+  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.
 - **Export image** (`lib/gameimage.ts` — the shared drawing module the render sidecar
   executes; the browser app no longer draws it): always the light palette (pinned constants
   mirroring `app.css`), the final board with classic axes A..O / 1..15, premium squares as