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/ARCHITECTURE.md

@@ -794,9 +794,14 @@ platform: the client calls the authenticated `game.export_url` op, the backend m
 **relative, HMAC-signed, short-lived path**
 (`/dl/{game}/{kind}?e=<expiry>&…&s=<HMAC-SHA256>`, 10-minute TTL,
 `BACKEND_EXPORT_SIGN_KEY`), and the client resolves it against its **own origin** (no
-service ever needs to know the public host) and hands it to the platform's native
-download: Telegram `downloadFile` (Bot API 8.0), `VKWebAppDownloadFile`, or a plain
-browser anchor elsewhere (including the desktop VK iframe). The GET is the gateway's
+service ever needs to know the public host) and hands it to the platform's most
+native affordance: Telegram `downloadFile` (Bot API 8.0; the chooser there is the
+native showPopup — the whole TG chain is activation-free bridge calls), VK's native
+photo viewer for the PNG (`VKWebAppShowImages`) + `VKWebAppDownloadFile` for the
+GCG, the OS share sheet with the fetched file on a mobile browser, or a plain
+anchor download (desktop, and the VK desktop iframe fallback). 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
 per-IP public rate limiter): the native download calls carry no cookies or headers, so
 the URL's signature — verified by the backend on its `/api/v1/public` group in constant

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

docs/FUNCTIONAL_ru.md

@@ -331,9 +331,10 @@ Telegram.
 **двух форматах за одной кнопкой 📤** — файл GCG и отрисованная **PNG-картинка**
 финальной позиции; экспорт доступен **только после завершения партии** и никогда —
 для тренировочной партии с ИИ (экспорт идущей партии раскрыл бы журнал ходов, а
-партия с ИИ одноразовая). Выбор формата — всегда **собственный модал** приложения,
-сознательно не нативный попап Telegram: его коллбек приходит без user activation и
-молча ломает доставку через share/буфер, к которой ведёт выбор.
+партия с ИИ одноразовая). Выбор формата — **нативный попап Telegram** внутри
+Telegram (там это безопасно: вся телеграмовская цепочка — bridge-вызовы, которым
+user activation не нужна) и собственный модал приложения в остальных случаях (у VK
+нативного чузера нет).
 
 **Картинка** рендерится на сервере (внутренний рендер-сайдкар исполняет тот же
 модуль отрисовки, что тестирует веб-клиент; всегда светлая тема): финальная
@@ -349,14 +350,16 @@ Telegram.
 и дата завершения в локали устройства. Типографика таблицы фиксирована; длинная
 партия растягивает доску (не ниже минимума), так что пустот на картинке нет.
 
-Доставка — **один путь для обоих форматов на любой платформе**: приложение
-запрашивает подписанную короткоживущую ссылку на скачивание и передаёт её родному
-механизму платформы — диалогу загрузки Telegram, `VKWebAppDownloadFile` у VK или
-обычному браузерному скачиванию файла в остальных случаях (включая десктопный
-iframe VK). Ссылка не требует логина при скачивании (родные загрузчики платформ его
-не несут), живёт минуты и отдаёт артефакт именованным файлом. 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
+Доставка — **одна подписанная короткоживущая ссылка для обоих форматов на любой
+платформе**, отданная самому родному механизму: диалог загрузки Telegram (обе
+платформы TG; поделиться дальше — из телеграмовского превью файла); в VK картинка
+открывается в **нативном просмотрщике фото VK** (сразу на экране — сохранение и
+шаринг штатными кнопками просмотрщика), а GCG идёт через загрузку VK; **мобильный
+браузер получает системную share-шторку** с файлом (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
 «картинка» не предлагается. Статистика (только у постоянных аккаунтов):
 победы, поражения, ничьи, макс. очков за партию и макс. очков за один ход (лучший
 ход, уже включающий все образованные им слова и бонус за все фишки). Также

docs/UI_DESIGN.md

@@ -406,15 +406,19 @@ 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**: always the app's own modal, never Telegram's native popup
-  (a `showPopup` callback carries no user activation, which silently breaks gesture-gated
-  delivery APIs — on-device finding). The accent button leads with the image, the GCG file
-  is the quiet alternative; both need the connection. Both formats travel the same
-  signed-URL route: the app mints a relative link (`game.export_url`), resolves it against
-  its own origin and hands it to the platform's native download — Telegram `downloadFile`,
-  `VKWebAppDownloadFile`, else a plain anchor download (also the desktop VK iframe, where
-  the bridge method is absent and the anchor works). A legacy Telegram client without
-  `downloadFile` keeps the old GCG clipboard copy and hides the image option. Confirming a
+  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
+  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,
+  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.
 - **Export image** (`lib/gameimage.ts` — the shared drawing module the render sidecar