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/README.md

@@ -29,7 +29,11 @@ pnpm codegen      # regenerate src/gen from edge.proto + scrabble.fbs (dev-time)
 gateway origin for a packaged (non-proxied) build. `VITE_TELEGRAM_BOT_ID` (Stage 11)
 enables the "Link Telegram" web sign-in (the Login Widget) — inert until the site
 domain is registered with BotFather (`/setdomain`); `VITE_TELEGRAM_LINK` is the
-share-to-Telegram deep-link base (Stage 9).
+share-to-Telegram deep-link base (Stage 9). `VITE_TELEGRAM_LINK_EN` / `VITE_TELEGRAM_LINK_RU`
+are the per-language "Play in Telegram" links shown on the landing page (Stage 17).
+
+The build has **two entries**: the game SPA (`index.html`, served at `/app/` and
+`/telegram/`) and a lightweight landing page (`landing.html`, served at `/`).
 
 ## How it talks to the gateway