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/landing.ts

@@ -0,0 +1,16 @@
+// Pure helpers for the public landing page (Stage 17), kept out of the Svelte component so
+// the per-language Telegram-bot link selection is unit-testable.
+
+import type { Locale } from './i18n/index.svelte';
+
+/**
+ * telegramBotLink returns the t.me link for the locale's game bot, or null when it is not
+ * configured. The two links are build-time vars (VITE_TELEGRAM_LINK_EN / VITE_TELEGRAM_LINK_RU)
+ * because the test and prod contours run different bots (different usernames), so the link
+ * cannot be hardcoded.
+ */
+export function telegramBotLink(locale: Locale): string | null {
+  const raw = locale === 'ru' ? import.meta.env.VITE_TELEGRAM_LINK_RU : import.meta.env.VITE_TELEGRAM_LINK_EN;
+  const link = (raw as string | undefined)?.trim();
+  return link ? link : null;
+}