feat(vk): embed the game as a VK Mini App

Mirror the Telegram Mini App wrapper for VK: the SPA loads at a new /vk/
entry, authenticates from VK's signed launch parameters, and provisions a
'vk' platform identity — the minimum to run the game in VK test mode.

- Gateway verifies the launch signature in-process (internal/vkauth:
  HMAC-SHA256 over the sorted vk_* params under GATEWAY_VK_APP_SECRET,
  base64url) — a pure offline check, no side-service. New auth.vk op
  (gated on the secret), backendclient.VKAuth, /vk/ SPA mount.
- Backend: KindVK + ProvisionVK/vkSeed, /sessions/vk handler, identity
  kind widened to include 'vk' (migration 00005, expand-contract).
- UI: src/lib/vk.ts (VK Bridge, lazy-imported), bootVK + the /vk/ boot
  dispatch, encodeVKLogin + authVK across transport/client/mock. VK omits
  the name from the signed params, so the client reads it via
  VKWebAppGetUserInfo as an unsigned display seed.
- Deploy: /vk in the edge Caddyfile, GATEWAY_VK_APP_SECRET wired through
  compose + .env.example + CI (TEST_) + prod-deploy (PROD_).
- Admin console: surface the VK user id (link to the VK profile) next to
  the Telegram id on the user card.
- Docs: ARCHITECTURE §12/§13, FUNCTIONAL (+ _ru), gateway README; VK
  integration reference under .claude/.

Signature algorithm verified against dev.vk.com plus independent Node/Python
references and a %2C edge-case vector.

ui/src/screens/BootError.svelte

@@ -1,8 +1,9 @@
 <script lang="ts">
   // The Mini App launch failed to authenticate after its silent retries (e.g. the backend was
-  // briefly down during a deploy). Inside Telegram there is no web login to fall back to, so this
-  // terminal screen offers a manual Retry that re-runs the launch (app.svelte retryTelegramBoot).
-  import { retryTelegramBoot } from '../lib/app.svelte';
+  // briefly down during a deploy). Inside a Mini App (Telegram or VK) there is no web login to fall
+  // back to, so this terminal screen offers a manual Retry that re-runs the launch (app.svelte
+  // retryMiniAppBoot).
+  import { retryMiniAppBoot } from '../lib/app.svelte';
   import { t } from '../lib/i18n/index.svelte';
 
   let retrying = $state(false);
@@ -10,7 +11,7 @@
     if (retrying) return;
     retrying = true;
     try {
-      await retryTelegramBoot();
+      await retryMiniAppBoot();
     } finally {
       retrying = false;
     }