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/.env.example

@@ -25,6 +25,8 @@ GM_BASICAUTH_HASH=                     # required; `caddy hash-password` bcrypt
 # --- UI build args (baked into the gateway image) ---------------------------
 VITE_TELEGRAM_BOT_ID=
 VITE_TELEGRAM_LINK=
+VITE_TELEGRAM_LINK_EN=                 # landing "Play in Telegram" link, English bot
+VITE_TELEGRAM_LINK_RU=                 # landing "Play in Telegram" link, Russian bot
 VITE_GATEWAY_URL=
 
 # --- Gateway ----------------------------------------------------------------