feat(export): server-rendered artifacts behind one signed download URL (#160)

The finished-game export (GCG + a new PNG of the final position) is one
signed, short-lived relative URL (game.export_url; HMAC-SHA256, 10-min
TTL, BACKEND_EXPORT_SIGN_KEY) resolved against the client's own origin
and delivered by the best affordance each platform has (five on-device
review rounds):

- TG Android/desktop: native showPopup chooser -> native downloadFile
  dialog (bridge-only chain, activation-safe).
- TG iOS: app-modal chooser -> OS share sheet with the fetched file
  (a popup callback cannot supply the activation the sheet needs).
- VK iOS: VKWebAppDownloadFile for both formats.
- VK Android: the PNG opens in VK's native image viewer, the GCG copies
  to the clipboard (the VK Android downloader hangs on any download,
  Content-Length/Range notwithstanding).
- VK desktop iframe / desktop browsers: plain anchor downloads.
- Mobile browsers: the OS share sheet (fetch-then-share).
- Legacy TG (< Bot API 8.0): app modal + GCG clipboard, no image option.

The PNG is rasterized on demand by the new internal `renderer` sidecar
(node:22-slim + skia-canvas + baked Liberation/Noto Color Emoji fonts)
executing the SAME ui/src/lib/gameimage.ts the ui project unit-tests;
the backend rebuilds the render payload from the journal +
engine.AlphabetTable, and the device date locale, IANA time zone and
localized non-play labels ride the signed URL. Nothing is stored — the
artifact re-derives from the immutable journal on each GET. The gateway
forwards /dl/* (caddy @gateway matcher extended) behind the per-IP
public rate limiter and serves bytes via http.ServeContent.

Deploy: renderer service in compose + prod overlay + rolling order +
prod push list; TEST_/PROD_EXPORT_SIGN_KEY secrets; the sidecar smoke
runs in the ui CI job. Docs: ARCHITECTURE, FUNCTIONAL(+_ru), UI_DESIGN,
TESTING, deploy/README, renderer/README.

ui/src/lib/client.ts

@@ -14,6 +14,7 @@ import type {
   FriendCode,
   GameList,
   GameView,
+  ExportUrl,
   GcgExport,
   History,
   HintResult,
@@ -161,6 +162,10 @@ export interface GatewayClient {
   profileUpdate(p: ProfileUpdate): Promise<Profile>;
   statsGet(): Promise<Stats>;
   exportGcg(gameId: string): Promise<GcgExport>;
+  /** Mints a signed download URL for a finished game's artifact. dateLocale and the
+   *  localized non-play labels (pass/exchange/resign/timeout, in that order) ride the
+   *  URL so the server-rendered image matches the player's presentation. */
+  exportUrl(gameId: string, kind: 'png' | 'gcg', dateLocale: string, actionLabels: string[], timeZone: string): Promise<ExportUrl>;
 
   // --- account linking & merge ---
   linkEmailRequest(email: string): Promise<void>;