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.test.ts

@@ -0,0 +1,20 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { telegramBotLink } from './landing';
+
+describe('telegramBotLink', () => {
+  afterEach(() => vi.unstubAllEnvs());
+
+  it('returns the per-language bot link when configured', () => {
+    vi.stubEnv('VITE_TELEGRAM_LINK_EN', 'https://t.me/Scrabble_Game');
+    vi.stubEnv('VITE_TELEGRAM_LINK_RU', 'https://t.me/Erudit_Game');
+    expect(telegramBotLink('en')).toBe('https://t.me/Scrabble_Game');
+    expect(telegramBotLink('ru')).toBe('https://t.me/Erudit_Game');
+  });
+
+  it('returns null when the locale link is unset or blank', () => {
+    vi.stubEnv('VITE_TELEGRAM_LINK_EN', '');
+    vi.stubEnv('VITE_TELEGRAM_LINK_RU', '   ');
+    expect(telegramBotLink('en')).toBeNull();
+    expect(telegramBotLink('ru')).toBeNull();
+  });
+});