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/social.spec.ts

@@ -154,14 +154,14 @@ test('GCG export appears only for a finished game', async ({ page }) => {
   // The finished game vs Kaya exposes export 📤 in the history header.
   await page.getByRole('button', { name: /Kaya/ }).click();
   await page.locator('.scoreboard').click(); // open the history
-  await expect(page.getByRole('button', { name: 'Export GCG' })).toBeVisible();
+  await expect(page.getByRole('button', { name: 'Export game' })).toBeVisible();
 });
 
 test('GCG export is hidden for an active game', async ({ page }) => {
   await loginLobby(page);
   await page.getByRole('button', { name: /Ann/ }).click();
   await page.locator('.scoreboard').click(); // open the history (shows 🏁 leave, not 📤 export)
-  await expect(page.getByRole('button', { name: 'Export GCG' })).toHaveCount(0);
+  await expect(page.getByRole('button', { name: 'Export game' })).toHaveCount(0);
 });
 
 test('finished game draws an inert footer and trims live-only controls', async ({ page }) => {
@@ -172,7 +172,7 @@ test('finished game draws an inert footer and trims live-only controls', async (
   await expect(page.locator('.tab').first()).toBeDisabled();
   // The history header offers Export GCG, not Drop game, once the game is over.
   await page.locator('.scoreboard').click();
-  await expect(page.getByRole('button', { name: 'Export GCG' })).toBeVisible();
+  await expect(page.getByRole('button', { name: 'Export game' })).toBeVisible();
   await expect(page.getByRole('button', { name: 'Drop game' })).toHaveCount(0);
   // The comms hub offers Chat only — the Dictionary tab is hidden for a finished game.
   await page.getByRole('button', { name: 'Chat' }).click(); // 💬