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/i18n/en.ts

@@ -310,7 +310,13 @@ export const en = {
   'stats.maxWord': 'Best move',
   'stats.guestHint': 'Sign in to track your statistics.',
 
-  'game.exportGcg': 'Export GCG',
+  'game.exportGcg': 'Export game',
+  'game.exportChoice': 'Export the finished game as:',
+  'game.exportImageOpt': 'Image (PNG)',
+  'game.exportGcgOpt': 'GCG file',
+  'game.imageSaveHint': 'Long-press (or right-click) the image to save it.',
+  'game.imageCopy': 'Copy image',
+  'game.imageCopied': 'Image copied to the clipboard.',
   'game.gcgActiveOnly': 'Available once the game is finished.',
   'game.gcgCopied': 'GCG copied to the clipboard.',
   'game.addFriendShort': 'Add friend?',