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.

.gitea/workflows/ci.yaml

@@ -267,6 +267,8 @@ jobs:
           TELEGRAM_TEST_ENV: "true"
           VITE_TELEGRAM_BOT_ID: ${{ vars.TEST_VITE_TELEGRAM_BOT_ID }}
           VITE_TELEGRAM_LINK: ${{ vars.TEST_VITE_TELEGRAM_LINK }}
+          VITE_TELEGRAM_LINK_EN: ${{ vars.TEST_VITE_TELEGRAM_LINK_EN }}
+          VITE_TELEGRAM_LINK_RU: ${{ vars.TEST_VITE_TELEGRAM_LINK_RU }}
           VITE_GATEWAY_URL: ${{ vars.TEST_VITE_GATEWAY_URL }}
           GATEWAY_DEFAULT_SUPPORTED_LANGUAGES: ${{ vars.TEST_GATEWAY_DEFAULT_SUPPORTED_LANGUAGES }}
           # Unset vars render empty -> the compose ":-" defaults apply.