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.ts

@@ -1,5 +1,5 @@
 // Pure helpers for the public landing page, kept out of the Svelte component so the
-// Telegram-channel link selection is unit-testable.
+// platform-entry link selection is unit-testable.
 
 /**
  * telegramChannelLink returns the t.me link for the single bot's game channel, or null
@@ -13,3 +13,15 @@ export function telegramChannelLink(): string | null {
   const name = (raw as string | undefined)?.trim().replace(/^@/, '');
   return name ? `https://t.me/${name}` : null;
 }
+
+/**
+ * vkAppLink returns the VK Mini App link for the landing "Play on VK" entry, or null when it is
+ * not configured. The link is a build-time var (VITE_VK_APP_LINK, the full URL
+ * `https://vk.com/app<id>`) because the test and prod contours run the same VK app but the var
+ * mirrors the per-contour Telegram entries.
+ */
+export function vkAppLink(): string | null {
+  const raw = import.meta.env.VITE_VK_APP_LINK;
+  const link = (raw as string | undefined)?.trim();
+  return link ? link : null;
+}