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.

docs/TESTING.md

@@ -19,8 +19,15 @@ tests or touching CI.
   the FlatBuffers codecs (friend list, invitation, stats), the win-rate
   derivation and the GCG share/copy/download choice, plus Playwright specs against the
   mock for the friends screen (code issue/redeem, accept a request), the lobby
-  invitations section, the stats screen, profile editing, and the GCG export's
-  finished-only visibility.
+  invitations section, the stats screen, profile editing, and the export chooser's
+  finished-only visibility + its signed-URL download flow (route-intercepted).
+- **Render sidecar** — `renderer/` (Node + skia-canvas executing the shared
+  `ui/src/lib/gameimage.ts`) carries a `node --test` smoke: the committed fixture (a
+  real self-played 35-move game) must rasterize to a plausible PNG
+  (`pnpm -C renderer test`). The drawing module's pure parts (scoresheet, layout,
+  notation) stay unit-tested in `ui/` — the sidecar test guards only the
+  skia/bundling seam. Pixel goldens are deliberately avoided (fonts differ across
+  hosts).
 - **Local-eval conformance** — the client's on-device move preview (the ported
   dawg reader + validator, `ui/src/lib/dict`) is checked byte-for-byte against the
   authoritative Go engine. `backend/cmd/dictgen` and `backend/cmd/validategen` emit