feat(landing): VK entry logo + restore the Telegram channel build-arg

Add the VK Mini App logo next to the Telegram one on the landing hero,
linked via the new VITE_VK_APP_LINK build-arg (full URL, wired through
compose, CI and prod-deploy from TEST_/PROD_VITE_VK_APP_LINK).

Also restore the landing's Telegram link itself: commit 57c778f
collapsed the per-language vars to VITE_TELEGRAM_GAME_CHANNEL_NAME in
compose/CI but left gateway/Dockerfile with the stale _EN/_RU ARGs and
without the plain one, so the built bundle saw the var as undefined and
dead-code-eliminated the whole channel-link branch — deployed landings
(prod included) have shown no Telegram logo since.

ui/src/lib/landing.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
-import { telegramChannelLink } from './landing';
+import { telegramChannelLink, vkAppLink } from './landing';
 
 describe('telegramChannelLink', () => {
   afterEach(() => vi.unstubAllEnvs());
@@ -14,3 +14,17 @@ describe('telegramChannelLink', () => {
     expect(telegramChannelLink()).toBeNull();
   });
 });
+
+describe('vkAppLink', () => {
+  afterEach(() => vi.unstubAllEnvs());
+
+  it('returns the configured VK app link verbatim', () => {
+    vi.stubEnv('VITE_VK_APP_LINK', 'https://vk.com/app54655464');
+    expect(vkAppLink()).toBe('https://vk.com/app54655464');
+  });
+
+  it('returns null when the link is unset or blank', () => {
+    vi.stubEnv('VITE_VK_APP_LINK', '   ');
+    expect(vkAppLink()).toBeNull();
+  });
+});