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.

gateway/internal/transcode/encode_social.go

@@ -251,6 +251,18 @@ func encodeInvitationList(r backendclient.InvitationListResp) []byte {
 	return b.FinishedBytes()
 }
 
+// encodeExportURL builds an ExportUrl payload.
+func encodeExportURL(r backendclient.ExportURLResp) []byte {
+	b := flatbuffers.NewBuilder(256)
+	path := b.CreateString(r.Path)
+	fn := b.CreateString(r.Filename)
+	fb.ExportUrlStart(b)
+	fb.ExportUrlAddPath(b, path)
+	fb.ExportUrlAddFilename(b, fn)
+	b.Finish(fb.ExportUrlEnd(b))
+	return b.FinishedBytes()
+}
+
 // encodeGcg builds a GcgExport payload.
 func encodeGcg(r backendclient.GcgResp) []byte {
 	b := flatbuffers.NewBuilder(1024)