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.

docs/FUNCTIONAL.md

@@ -322,9 +322,9 @@ Finished games are archived in a dictionary-independent form and exportable in
 **two formats behind one 📤 button** — the GCG file and a rendered **PNG image** of
 the final position; the export is offered **only once a game is finished**, and
 never for an honest-AI practice game (a live game's export would leak the move
-journal; an AI game is throwaway). The format chooser is always the app's **own
-modal** — deliberately not Telegram's native popup, whose callback runs without
-user activation and silently breaks the share/clipboard delivery it leads to.
+journal; an AI game is throwaway). The format chooser is Telegram's **native popup**
+inside Telegram (safe there: the whole Telegram chain is bridge calls, which need no
+user activation) and the app's own modal elsewhere (VK has no native chooser).
 
 The **image** is rendered on the server (the internal render sidecar runs the same
 drawing module the web client tests; always the light theme): the final board with
@@ -340,15 +340,17 @@ running totals alone do not include it). The footer stamps the site host and the
 finish date in the device locale. The scoresheet typography is fixed; a long game
 stretches the board (never below its minimum) so the image carries no dead space.
 
-Delivery is **one route for both formats on every platform**: the app requests a
-signed, short-lived download link and hands it to the platform's native download —
-Telegram's download dialog, VK's `VKWebAppDownloadFile`, or an ordinary browser
-file download elsewhere (including the desktop VK iframe). The link needs no login
-to fetch (the platforms' download calls carry none), is valid for minutes, and
-serves the artifact as a named file attachment. The single exception is a legacy
-Telegram client without the download dialog (pre-Bot API 8.0): there the GCG falls
-back to the old clipboard copy (with the confirming toast) and the image option is
-not offered. Statistics (durable accounts only):
+Delivery is **one signed, short-lived link for both formats on every platform**,
+handed to the most native affordance: Telegram's download dialog (both TG
+platforms; sharing onwards from Telegram's own file preview); on VK the image opens
+in **VK's native photo viewer** (on screen at once — saving/sharing are the
+viewer's own controls) and the GCG goes through VK's download; a **mobile browser
+gets the OS share sheet** with the file (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 client without the download dialog (pre-Bot API 8.0): there
+the GCG falls back to the old clipboard copy (with the confirming toast) and the
+image option is not offered. Statistics (durable accounts only):
 wins, losses, draws, max points in a game, and max points for a single move (the
 best play, which already includes every word it formed plus the all-tiles bonus). It
 also shows the player's **move count** (their plays — passes and exchanges do not