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/en.ts

@@ -152,6 +152,10 @@ export const en = {
   'about.description': 'A multiplatform Scrabble game.',
   'about.version': 'Version {v}',
 
+  'landing.tagline': 'Play Scrabble with friends or a smart robot — in your browser or on Telegram.',
+  'landing.playWeb': 'Play in browser',
+  'landing.playTelegram': 'Play in Telegram',
+
   'lang.en': 'English',
   'lang.ru': 'Русский',
 

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',
 

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();
+  });
+});

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;
+}