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.

docs/UI_DESIGN.md

@@ -406,29 +406,36 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
 - **History / export**: the in-game slide-down history lays each move out in a per-seat grid
   (the word(s) and the move score, no running total); the 📤 in the history header appears
   only once the game is finished — never in an honest-AI game (throwaway practice) — and
-  opens the **format chooser**: always the app's own modal, never Telegram's native popup
-  (a `showPopup` callback carries no user activation, which silently breaks the
-  `navigator.share` / clipboard delivery it leads to — on-device finding). The accent button
-  leads with the image where offered, the GCG file is the quiet alternative (GCG needs the
-  connection, the image renders locally). The GCG file goes by Web Share where available, a
-  clipboard copy in an Android in-app WebView (Telegram / VK, which has neither Web Share nor
-  a working download), else a Blob download. The image option shows **only outside the in-app
-  WebViews** for now (a binary PNG has no working route there — no Web Share, a dead
-  `<a download>`, a text-only clipboard, and the long-press menu mangles data: URLs); it
-  returns there with the server-rendered signed-URL delivery. Confirming a resign reveals
-  the full board: it closes the history drawer (portrait) and zooms the board out.
-- **Export image** (`lib/gameimage.ts`, Canvas 2D, dynamically imported): always the light
-  palette (pinned constants mirroring `app.css`), the final board with classic axes A..O /
-  1..15, premium squares as plain colour fills (no text labels), tiles in the in-game style
-  (bevel, letter top-left, value bottom-right, ✻ for an Erudit blank), no last-move
-  highlight. The right column is the scoresheet: per-seat name over the final score (🏆 by
-  the winner), one fixed-typography row per move — muted classic coordinate (across =
-  row-first `8G`, down = column-first `H8`), the main word, points right-aligned; extra
-  words of a multi-word play on a smaller second line; italic localized notes for
-  pass/exchange/resign/timeout; a closing muted ± row for the endgame rack settlement when
-  present. Footer: `hostname · finish date` in the **device** locale. The scoresheet's
-  height drives the board side (never below its minimum): a long game stretches the board,
-  never the typography, so the image has no dead space.
+  opens the **format chooser**: Telegram's native popup on TG Android/desktop — safe
+  there, since that chain is all bridge calls needing no user activation (a popup callback
+  must never lead into `navigator.share`/clipboard, the on-device finding) — and the app's
+  own modal elsewhere (TG iOS delivers via the OS share sheet, which needs the modal
+  click's activation; VK has no native chooser; the accent button leads with the image,
+  both options need the connection). Both formats mint the same signed relative link
+  (`game.export_url`), resolved against the app's own origin, then delivered per platform
+  (each branch owner-verified on-device): TG Android/desktop `downloadFile` (its preview
+  shares onwards); TG iOS the OS share sheet with the fetched file; VK iOS
+  `VKWebAppDownloadFile` for both formats (VK's native share flow); VK Android — whose
+  DownloadFile hangs — the PNG in VK's native image viewer (`VKWebAppShowImages`, its save
+  works) and the GCG to the clipboard; the VK desktop iframe plain anchor downloads; a
+  mobile browser the OS share sheet (the proven fetch-then-share pattern); a desktop
+  browser an anchor download. A legacy Telegram
+  client without `downloadFile` keeps the old GCG clipboard copy, hides the image option
+  and stays on the app modal. Confirming a resign reveals the full board: it closes the
+  history drawer (portrait) and zooms the board out.
+- **Export image** (`lib/gameimage.ts` — the shared drawing module the render sidecar
+  executes; the browser app no longer draws it): always the light palette (pinned constants
+  mirroring `app.css`), the final board with classic axes A..O / 1..15, premium squares as
+  plain colour fills (no text labels), tiles in the in-game style (bevel, letter top-left,
+  value bottom-right, ✻ for an Erudit blank), no last-move highlight. The right column is
+  the scoresheet: per-seat name over the final score (🏆 by the winner), one
+  fixed-typography row per move — muted classic coordinate (across = row-first `8G`, down =
+  column-first `H8`), the main word, points right-aligned; extra words of a multi-word play
+  on a smaller second line; italic localized notes for pass/exchange/resign/timeout; a
+  closing muted ± row for the endgame rack settlement when present. Footer:
+  `hostname · finish date` in the **device** locale. The scoresheet's height drives the
+  board side (never below its minimum): a long game stretches the board, never the
+  typography, so the image has no dead space.
 - **Finished game**: the board keeps no last-word highlight and no zoom; the history header
   offers *Export game* (not *Drop game*; an AI game offers neither) and the comms hub hides the 🔎 *Dictionary* tab — and a **finished AI game has no comms at all** (no chat, dictionary closed), so its 💬 entry is dropped from the header too; and
   the footer (rack + tab bar) is **drawn but inert** (greyed, non-interactive) rather than