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/vk.ts

@@ -155,6 +155,36 @@ export async function vkShare(link: string): Promise<boolean> {
   }
 }
 
+/**
+ * vkShowImages opens VK's native image viewer on url — on the Android client this is the
+ * PNG delivery: the viewer's own "save" works there, while VKWebAppDownloadFile hangs
+ * indefinitely (on-device finding). Resolves false on any failure, so the caller can
+ * fall back.
+ */
+export async function vkShowImages(url: string): Promise<boolean> {
+  try {
+    await (await bridge()).send('VKWebAppShowImages', { images: [url] });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * vkDownloadFile downloads url as filename through the VK client (VKWebAppDownloadFile) —
+ * the mobile in-app file delivery, where the webview ignores <a download>. Resolves false
+ * on any failure or where the method is unavailable (notably the desktop iframe), so the
+ * caller falls back to a plain browser download of the same URL.
+ */
+export async function vkDownloadFile(url: string, filename: string): Promise<boolean> {
+  try {
+    await (await bridge()).send('VKWebAppDownloadFile', { url, filename });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 /**
  * vkCopyText copies text to the clipboard via VKWebAppCopyText — which works inside the VK iframe,
  * where navigator.clipboard is blocked. Resolves true on success, false on any failure or outside VK.