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.

CLAUDE.md

@@ -125,6 +125,7 @@ gateway/                     # module scrabble/gateway: Connect-RPC edge, embeds
 ui/                          # Svelte + Vite SPA + landing (Node project, not in go.work)
 pkg/                         # shared: telemetry, version, wire/FlatBuffers, proto, mtls
 platform/telegram/           # Telegram side-service: cmd/validator (HMAC, no VPN) + cmd/bot (Bot API; dials gateway over reverse mTLS bot-link)
+renderer/                    # image-render sidecar (Node + skia-canvas): runs ui/src/lib/gameimage.ts server-side for the finished-game PNG export
 loadtest/                    # module scrabble/loadtest: the load/stress harness
 docs/  .gitea/workflows/  CLAUDE.md  README.md
 backend/Dockerfile gateway/Dockerfile platform/telegram/Dockerfile loadtest/Dockerfile  # multi-stage distroless; gateway/Dockerfile has the `landing` target, platform/telegram/Dockerfile has `validator`+`bot` targets