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.

ui/src/lib/history.ts

@@ -16,11 +16,26 @@ export interface HistoryCell {
   words?: string[];
   /** Points scored by the move — set for kind 'play'. */
   score?: number;
+  /** The main word's board coordinate in classic notation (see moveCoord) — set for kind
+   * 'play'. The in-game panel does not show it; the game-image export does. */
+  coord?: string;
   /** The non-play action ('pass' | 'exchange' | 'resign' | 'timeout' | …) — set for kind
    * 'action'; the component localizes it. */
   action?: string;
 }
 
+/**
+ * moveCoord renders a play's board coordinate in the classic notation the GCG export uses
+ * (the backend's gcgPos): row-then-column (e.g. "8G") for an across play, column-then-row
+ * ("H8") for a down play. Rows are 1-based numbers top to bottom, columns letters from "A"
+ * left to right.
+ */
+export function moveCoord(mv: Pick<MoveRecord, 'dir' | 'mainRow' | 'mainCol'>): string {
+  const col = String.fromCharCode(65 + mv.mainCol);
+  const row = String(mv.mainRow + 1);
+  return mv.dir === 'V' ? col + row : row + col;
+}
+
 /**
  * historyGrid arranges the move journal into a row-major matrix with one column per seat.
  * Each seat's moves fill its column top to bottom, so a seat's k-th move lands in row k of
@@ -40,7 +55,7 @@ export function historyGrid(
     if (m.player < 0 || m.player >= seatCount) continue; // defensive: drop an out-of-range seat
     cols[m.player].push(
       m.action === 'play'
-        ? { kind: 'play', player: m.player, words: m.words, score: m.score }
+        ? { kind: 'play', player: m.player, words: m.words, score: m.score, coord: moveCoord(m) }
         : { kind: 'action', player: m.player, action: m.action },
     );
   }