feat(game): finished-game export as a PNG image behind a format chooser

The history header's export button now opens a chooser — Telegram's
native popup inside Telegram, the app's own modal elsewhere — offering
the GCG file and a new client-rendered PNG of the final position
(lib/gameimage, Canvas 2D, lazy dynamic import, zero dependencies):
light theme, classic A..O/1..15 axes, label-free premium fills, and a
fixed-typography per-seat scoresheet with GCG-style move coordinates,
multi-word sub-lines, endgame rack-settlement row, winner trophy and a
hostname + device-locale finish date footer; a long game stretches the
board, never the typography.

Delivery mirrors the GCG rules (Web Share with no blob fallback, else
download) except on Android Telegram/VK WebViews and the desktop VK
iframe, where a binary PNG has no clipboard-text fallback: those get a
preview modal with a long-press/right-click save hint and a copy-image
button where ClipboardItem exists.

docs/UI_DESIGN.md

@@ -403,15 +403,32 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
   lobby with a calm modal pointing at the bot (`@<username>`), not a red error on the
   Friends tab. Flex text inputs carry `min-width:0` so they shrink instead of overflowing in
   Safari.
-- **History / GCG**: 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); *Export GCG* (the 📤 in the history
-  header) delivers the `.gcg` file — 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 —
-  and appears only once the game is finished — and
-  never in an honest-AI game (throwaway practice). Confirming a resign reveals the full board:
-  it closes the history drawer (portrait) and zooms the board out.
+- **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
+  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 /
+  1..15, premium squares as plain colour fills (no text labels), tiles in the in-game style
+  (bevel, letter top-left, value bottom-right, ✻ for an Erudit blank), no last-move
+  highlight. The right column is the scoresheet: per-seat name over the final score (🏆 by
+  the winner), one fixed-typography row per move — muted classic coordinate (across =
+  row-first `8G`, down = column-first `H8`), the main word, points right-aligned; extra
+  words of a multi-word play on a smaller second line; italic localized notes for
+  pass/exchange/resign/timeout; a closing muted ± row for the endgame rack settlement when
+  present. Footer: `hostname · finish date` in the **device** locale. The scoresheet's
+  height drives the board side (never below its minimum): a long game stretches the board,
+  never the typography, so the image has no dead space.
 - **Finished game**: the board keeps no last-word highlight and no zoom; the history header
-  offers *Export GCG* (not *Drop game*; an AI game offers neither) and the comms hub hides the 🔎 *Dictionary* tab — and a **finished AI game has no comms at all** (no chat, dictionary closed), so its 💬 entry is dropped from the header too; and
+  offers *Export game* (not *Drop game*; an AI game offers neither) and the comms hub hides the 🔎 *Dictionary* tab — and a **finished AI game has no comms at all** (no chat, dictionary closed), so its 💬 entry is dropped from the header too; and
   the footer (rack + tab bar) is **drawn but inert** (greyed, non-interactive) rather than
   hidden, so the layout does not jump. Chat send / nudge are the ⬆️ / 🛎️ icons. The input
   row is turn-driven: on your turn the message field shows until your one allowed message is