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/backendclient/api_social.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"net/http"
 	"net/url"
+	"strings"
 )
 
 // The response structs and client methods mirror the backend's social,
@@ -347,3 +348,28 @@ func (c *Client) ExportGCG(ctx context.Context, userID, gameID string) (GcgResp,
 	err := c.do(ctx, http.MethodGet, c.gamePath(gameID, "/gcg"), userID, "", nil, &out)
 	return out, err
 }
+
+// ExportURLResp is the minted signed download link for an export artifact.
+type ExportURLResp struct {
+	Path     string `json:"path"`
+	Filename string `json:"filename"`
+}
+
+// ExportURL mints a signed download URL for a finished game's artifact (kind
+// "png" or "gcg"). dateLocale and the localized non-play labels ride the URL so
+// the server render matches the player's presentation.
+func (c *Client) ExportURL(ctx context.Context, userID, gameID, kind, dateLocale, timeZone string, labels []string) (ExportURLResp, error) {
+	q := url.Values{"kind": {kind}}
+	if dateLocale != "" {
+		q.Set("dl", dateLocale)
+	}
+	if timeZone != "" {
+		q.Set("tz", timeZone)
+	}
+	if len(labels) > 0 {
+		q.Set("t", strings.Join(labels, ","))
+	}
+	var out ExportURLResp
+	err := c.do(ctx, http.MethodGet, c.gamePath(gameID, "/export-url")+"?"+q.Encode(), userID, "", nil, &out)
+	return out, err
+}