feat(export): server-rendered artifacts behind one signed download URL (#160)

The finished-game export (GCG + a new PNG of the final position) is one
signed, short-lived relative URL (game.export_url; HMAC-SHA256, 10-min
TTL, BACKEND_EXPORT_SIGN_KEY) resolved against the client's own origin
and delivered by the best affordance each platform has (five on-device
review rounds):

- TG Android/desktop: native showPopup chooser -> native downloadFile
  dialog (bridge-only chain, activation-safe).
- TG iOS: app-modal chooser -> OS share sheet with the fetched file
  (a popup callback cannot supply the activation the sheet needs).
- VK iOS: VKWebAppDownloadFile for both formats.
- VK Android: the PNG opens in VK's native image viewer, the GCG copies
  to the clipboard (the VK Android downloader hangs on any download,
  Content-Length/Range notwithstanding).
- VK desktop iframe / desktop browsers: plain anchor downloads.
- Mobile browsers: the OS share sheet (fetch-then-share).
- Legacy TG (< Bot API 8.0): app modal + GCG clipboard, no image option.

The PNG is rasterized on demand by the new internal `renderer` sidecar
(node:22-slim + skia-canvas + baked Liberation/Noto Color Emoji fonts)
executing the SAME ui/src/lib/gameimage.ts the ui project unit-tests;
the backend rebuilds the render payload from the journal +
engine.AlphabetTable, and the device date locale, IANA time zone and
localized non-play labels ride the signed URL. Nothing is stored — the
artifact re-derives from the immutable journal on each GET. The gateway
forwards /dl/* (caddy @gateway matcher extended) behind the per-IP
public rate limiter and serves bytes via http.ServeContent.

Deploy: renderer service in compose + prod overlay + rolling order +
prod push list; TEST_/PROD_EXPORT_SIGN_KEY secrets; the sidecar smoke
runs in the ui CI job. Docs: ARCHITECTURE, FUNCTIONAL(+_ru), UI_DESIGN,
TESTING, deploy/README, renderer/README.

ui/src/lib/share.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
-import { pickGcgDelivery, pickImageDelivery, pickTextShare, shareOrDownloadGcg, shareOrDownloadImage, shareText } from './share';
+import { downloadUrl, pickGcgDelivery, pickTextShare, shareOrDownloadGcg, shareText } from './share';
 import type { GcgExport } from './model';
 
 const file = {} as File;
@@ -83,52 +83,21 @@ describe('shareOrDownloadGcg', () => {
   });
 });
 
-describe('pickImageDelivery', () => {
-  const canShareNav = { canShare: () => true, share: async () => {} };
-  const noShareNav = { canShare: () => false, share: async () => {} };
-
-  it('shares when the platform can share files', () => {
-    expect(pickImageDelivery(canShareNav, file)).toBe('share');
-  });
-
-  it('downloads on a plain browser without Web Share (desktop)', () => {
-    expect(pickImageDelivery(noShareNav, file)).toBe('download');
-    expect(pickImageDelivery(undefined, file)).toBe('download');
-  });
-});
-
-describe('shareOrDownloadImage', () => {
+describe('downloadUrl', () => {
   afterEach(() => vi.unstubAllGlobals());
 
-  function stubEnv(canShare: boolean, share: () => Promise<void>) {
+  it('clicks a temporary anchor carrying the URL and the filename', () => {
     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())).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())).toBe('downloaded');
+    downloadUrl('https://example.test/dl/g-1/png?e=1&s=x', 'game-1.png');
 
     expect(createElement).toHaveBeenCalledWith('a');
-    expect(anchor.click).toHaveBeenCalledOnce();
+    expect(anchor.href).toBe('https://example.test/dl/g-1/png?e=1&s=x');
     expect(anchor.download).toBe('game-1.png');
+    expect(anchor.click).toHaveBeenCalledOnce();
+    expect(anchor.remove).toHaveBeenCalledOnce();
   });
 });