fix(game): own export chooser modal; PNG option outside in-app webviews only

Telegram's native showPopup delivers its callback with no user
activation, so navigator.share (TG iOS) and clipboard writes (TG
Android GCG copy) silently fail from it — the chooser is now always the
app's own modal, keeping the button click's gesture alive for the
delivery APIs.

The data:URL preview modal is dropped: the Android TG/VK long-press
menu mangles data: URLs (dead download, black-screen open, base64
clipboard garbage), so a binary PNG has no working client-side route in
those webviews at all. The image option is withheld there until the
server-rendered signed-URL delivery (Telegram downloadFile /
VKWebAppDownloadFile) lands; the plain web and mobile browsers keep it.

On-device findings by the owner on the test contour (TG iOS, TG
Android, VK Android).

docs/UI_DESIGN.md

@@ -406,14 +406,16 @@ 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 a **format chooser**: Telegram's native popup inside Telegram (image / GCG / cancel),
-  the app's own modal elsewhere (the accent button leads with the image, the GCG file is the
-  quiet alternative; GCG needs the connection, the image renders locally). The GCG file goes
-  by Web Share where available, a clipboard copy in an Android in-app WebView (Telegram / VK,
-  which has neither Web Share nor a working download), else a Blob download. The image goes by
-  Web Share / Blob download too, but on those in-app WebViews (and the desktop VK iframe) it
-  opens a **preview modal** instead — the PNG as a data: URL with a long-press / right-click
-  save hint and a copy-image button where `ClipboardItem` exists. Confirming a resign reveals
+  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 the
+  `navigator.share` / clipboard delivery it leads to — on-device finding). The accent button
+  leads with the image where offered, the GCG file is the quiet alternative (GCG needs the
+  connection, the image renders locally). The GCG file goes by Web Share where available, a
+  clipboard copy in an Android in-app WebView (Telegram / VK, which has neither Web Share nor
+  a working download), else a Blob download. The image option shows **only outside the in-app
+  WebViews** for now (a binary PNG has no working route there — no Web Share, a dead
+  `<a download>`, a text-only clipboard, and the long-press menu mangles data: URLs); it
+  returns there with the server-rendered signed-URL delivery. Confirming a resign reveals
   the full board: it closes the history drawer (portrait) and zooms the board out.
 - **Export image** (`lib/gameimage.ts`, Canvas 2D, dynamically imported): always the light
   palette (pinned constants mirroring `app.css`), the final board with classic axes A..O /