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/share.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
-import { pickGcgDelivery, pickTextShare, shareOrDownloadGcg, shareText } from './share';
+import { pickGcgDelivery, pickImageDelivery, pickTextShare, shareOrDownloadGcg, shareOrDownloadImage, shareText } from './share';
 import type { GcgExport } from './model';
 
 const file = {} as File;
@@ -83,6 +83,69 @@ describe('shareOrDownloadGcg', () => {
   });
 });
 
+describe('pickImageDelivery', () => {
+  const canShareNav = { canShare: () => true, share: async () => {} };
+  const noShareNav = { canShare: () => false, share: async () => {} };
+
+  it('shares when the platform can share files, even in an in-app webview (iOS)', () => {
+    expect(pickImageDelivery(canShareNav, file, false)).toBe('share');
+    expect(pickImageDelivery(canShareNav, file, true)).toBe('share');
+  });
+
+  it('previews in an in-app webview that cannot share files (a binary PNG has no clipboard-text fallback)', () => {
+    expect(pickImageDelivery(noShareNav, file, true)).toBe('preview');
+    expect(pickImageDelivery(undefined, file, true)).toBe('preview');
+  });
+
+  it('downloads on a plain browser without Web Share (desktop)', () => {
+    expect(pickImageDelivery(noShareNav, file, false)).toBe('download');
+    expect(pickImageDelivery(undefined, file, false)).toBe('download');
+  });
+});
+
+describe('shareOrDownloadImage', () => {
+  afterEach(() => vi.unstubAllGlobals());
+
+  function stubEnv(canShare: boolean, share: () => Promise<void>) {
+    const anchor = { href: '', download: '', click: vi.fn(), remove: vi.fn() };
+    const createElement = vi.fn(() => anchor);
+    vi.stubGlobal('navigator', { canShare: () => canShare, share });
+    vi.stubGlobal('document', { createElement, body: { appendChild: vi.fn() } });
+    vi.stubGlobal('URL', { createObjectURL: vi.fn(() => 'blob:x'), revokeObjectURL: vi.fn() });
+    return { anchor, createElement };
+  }
+
+  const png = () => ({ name: 'game-1.png', type: 'image/png' }) as File;
+
+  it('never falls back to the navigating Blob download when a share is cancelled', async () => {
+    const share = vi.fn().mockRejectedValue(new DOMException('cancelled', 'AbortError'));
+    const { createElement } = stubEnv(true, share);
+
+    expect(await shareOrDownloadImage(png(), false)).toBe('shared');
+
+    expect(share).toHaveBeenCalledOnce();
+    expect(createElement).not.toHaveBeenCalled();
+  });
+
+  it('downloads via an anchor on a desktop browser that cannot share files', async () => {
+    const { anchor, createElement } = stubEnv(false, vi.fn());
+
+    expect(await shareOrDownloadImage(png(), false)).toBe('downloaded');
+
+    expect(createElement).toHaveBeenCalledWith('a');
+    expect(anchor.click).toHaveBeenCalledOnce();
+    expect(anchor.download).toBe('game-1.png');
+  });
+
+  it('hands an in-app webview over to the preview modal (no share, no dead download)', async () => {
+    const { createElement } = stubEnv(false, vi.fn());
+
+    expect(await shareOrDownloadImage(png(), true)).toBe('preview');
+
+    expect(createElement).not.toHaveBeenCalled();
+  });
+});
+
 describe('pickTextShare', () => {
   it('shares when Web Share is available', () => {
     expect(pickTextShare({ share: async () => {}, canShare: () => true })).toBe('share');