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/vite.config.ts

@@ -1,3 +1,4 @@
+import { resolve } from 'node:path';
 import { defineConfig } from 'vite';
 import { svelte } from '@sveltejs/vite-plugin-svelte';
 
@@ -34,5 +35,14 @@ export default defineConfig(({ mode }) => ({
   build: {
     target: 'es2022',
     sourcemap: true,
+    // Two entries (Stage 17): the game SPA (index.html, served at /app/ + /telegram/) and the
+    // public landing page (landing.html, served at /). Assets are shared in dist/assets/, and
+    // the relative base lets one build serve under any path.
+    rollupOptions: {
+      input: {
+        main: resolve(import.meta.dirname, 'index.html'),
+        landing: resolve(import.meta.dirname, 'landing.html'),
+      },
+    },
   },
 }));