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.

ui/src/lib/i18n/ru.ts

@@ -153,6 +153,10 @@ export const ru: Record<MessageKey, string> = {
   'about.description': 'Мультиплатформенная игра в скрабл.',
   'about.version': 'Версия {v}',
 
+  'landing.tagline': 'Играй в Скрэббл с друзьями или умным роботом — в браузере или в Telegram.',
+  'landing.playWeb': 'Играть в браузере',
+  'landing.playTelegram': 'Играть в Telegram',
+
   'lang.en': 'English',
   'lang.ru': 'Русский',