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

The finished-game export now works identically on every platform: the
client mints a signed relative URL (game.export_url) carrying its date
locale, IANA time zone and localized non-play labels, resolves it
against its own origin and hands it to the platform's native download —
Telegram downloadFile, VKWebAppDownloadFile, or a plain browser anchor
(also the desktop VK iframe). Both artifacts ride the route: the .gcg
text (no more clipboard mode, except the legacy pre-8.0 Telegram
fallback) and the PNG of the final position.

The PNG is rasterized by a new internal 'renderer' sidecar (node:22-slim
+ skia-canvas + baked Liberation/Noto Color Emoji fonts) executing the
SAME ui/src/lib/gameimage.ts the web project unit-tests, bundled at
image build time — one renderer, no drift; the browser no longer draws
or delivers bytes itself. The backend rebuilds the render payload from
the journal + engine.AlphabetTable, verifies the HMAC (10-minute TTL,
BACKEND_EXPORT_SIGN_KEY, constant-time, uniform 404s) on its public
group, and streams the artifact as a named attachment; the gateway
forwards /dl/* behind the per-IP public rate limiter (caddy @gateway
matcher extended — the landing catch-all trap).

Deploy: renderer service (compose + prod overlay + roll before backend
+ prod push list), EXPORT_SIGN_KEY env (TEST_/PROD_ secrets), CI runs
the sidecar smoke in the ui job. Docs: ARCHITECTURE export-delivery
section, FUNCTIONAL(+_ru), UI_DESIGN, TESTING, deploy/README.

ui/src/lib/transport.ts

@@ -271,6 +271,11 @@ export function createTransport(baseUrl: string): GatewayClient {
     async exportGcg(gameId) {
       return codec.decodeGcg(await exec('game.gcg', codec.encodeGameAction(gameId)));
     },
+    async exportUrl(gameId, kind, dateLocale, actionLabels, timeZone) {
+      return codec.decodeExportUrl(
+        await exec('game.export_url', codec.encodeExportUrlRequest(gameId, kind, dateLocale, actionLabels, timeZone)),
+      );
+    },
 
     subscribe(onEvent, onError) {
       const ctrl = new AbortController();