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/e2e/quickmatch.spec.ts

@@ -80,7 +80,7 @@ test('AI game: after it ends, no GCG export and no comms entry', async ({ page }
   // Reopen the history (resigning auto-closed it): the finished AI game offers no GCG export and
   // no comms entry at all.
   await page.locator('.scoreboard').click();
-  await expect(page.getByRole('button', { name: 'Export GCG' })).toHaveCount(0);
+  await expect(page.getByRole('button', { name: 'Export game' })).toHaveCount(0);
   await expect(page.getByRole('button', { name: 'Chat' })).toHaveCount(0);
 });