Stage 17 round 6 (#16-20): landing page, /app/ move, cache + stream fixes

Close out Stage 17 round 6:

- Landing page at / — one Vite build with two entries (index.html = game
  SPA, landing.html = a lightweight landing reusing the theme/i18n/
  aboutContent leaf modules, not the app store).
- Move the web game SPA to /app/; the Telegram Mini App stays at /telegram/
  (gateway webui.Handler(stripPrefix, indexName): landing at /, SPA at /app/
  + /telegram/). Per-language "Play in Telegram" link via new
  VITE_TELEGRAM_LINK_EN/_RU build vars (button hides when unset).
- Cache headers: hash-named /assets/* immutable, HTML shells no-cache (the
  go:embed zero modtime emitted no validators, so the client re-downloaded
  the whole bundle every launch).
- Live-stream 15s abort fix: an immediate heartbeat on open + a 10s default
  interval (the first tick at 15s raced the edge idle timeout -> reconnect
  storm).

PLAN/ARCHITECTURE(§13)/FUNCTIONAL(+ru)/gateway+ui+deploy READMEs updated;
round 6 closed. Tests: gateway webui/connectsrv units, ui landing unit + e2e,
full e2e (60) green.

deploy/README.md

@@ -12,7 +12,7 @@ operational reference for **every environment variable**.
 | Service | Image | Role |
 | --- | --- | --- |
 | `caddy` | `caddy:2-alpine` | Edge proxy (alias `scrabble` on `edge`): single `/_gm` Basic-Auth → admin console + Grafana; everything else → gateway. TLS per `CADDY_SITE_ADDRESS`. |
-| `gateway` | built (`gateway/Dockerfile`) | Public edge; serves the embedded SPA at `/` and `/telegram/`; Connect-RPC edge. |
+| `gateway` | built (`gateway/Dockerfile`) | Public edge; serves the embedded landing at `/` and the game SPA at `/app/` + `/telegram/`; Connect-RPC edge. |
 | `backend` | built (`backend/Dockerfile`) | Domain service; bakes in the DAWG dictionaries; runs migrations at boot. |
 | `postgres` | `postgres:17-alpine` | Database (named volume, `pg_isready` healthcheck). |
 | `vpn` + `telegram` | sidecar + built (`platform/telegram/Dockerfile`) | Telegram connector; egresses through the AmneziaWG sidecar; internal gRPC at `telegram:9091`. |
@@ -84,9 +84,11 @@ connector **fails at boot** if both are empty.
 | `GATEWAY_DEFAULT_SUPPORTED_LANGUAGES` | variable | `en,ru` | Variant-gating set for non-Telegram logins (web/email/guest). |
 | `VITE_TELEGRAM_BOT_ID` | variable | _(empty)_ | UI build-arg: numeric bot id for the web Login Widget. |
 | `VITE_TELEGRAM_LINK` | variable | _(empty)_ | UI build-arg: deep-link base for share-to-Telegram (e.g. `https://t.me/<bot>/<app>`). |
+| `VITE_TELEGRAM_LINK_EN` | variable | _(empty)_ | UI build-arg: the landing "Play in Telegram" link for the **English** bot (e.g. `https://t.me/Scrabble_Game`). |
+| `VITE_TELEGRAM_LINK_RU` | variable | _(empty)_ | UI build-arg: the landing "Play in Telegram" link for the **Russian** bot (e.g. `https://t.me/Erudit_Game`). |
 | `VITE_GATEWAY_URL` | variable | _(empty)_ | UI build-arg: gateway origin; empty = same-origin (the usual single-origin deploy). |
 
-The three `VITE_*` are **build-args** baked into the gateway image at build time, so
+The five `VITE_*` are **build-args** baked into the gateway image at build time, so
 changing them requires a rebuild (`--build`), not just a restart.
 
 ## Fixed internal wiring (not operator-set)