feat(account): VK ID web login to link a VK identity from a browser

A browser has no signed VK Mini App launch params, so linking VK on the web uses
VK ID's raw OAuth 2.1 flow (PKCE, no @vkid/sdk): the SPA redirects to VK's hosted
login and returns with an authorization code, which the gateway exchanges
server-side (confidential, under the VK "Web" app's protected key) for the trusted
vk user id — then the existing link/merge machinery attaches or merges it.

- fbs LinkVKRequest{code, device_id, code_verifier}; codec + TS bindings.
- backend link.Service ConfirmVK/MergeVK/attachVK (KindVK, mirror Telegram),
  handleLinkVK[Merge], routes /user/link/vk[/merge], backendclient LinkVK[Merge].
- gateway internal/vkid confidential code exchange (id.vk.com/oauth2/auth);
  transcode link.vk.confirm/merge (registered only when configured) + config
  GATEWAY_VK_ID_{APP_ID,CLIENT_SECRET,REDIRECT_URL} + main wiring.
- UI lib/vkid (PKCE authorize redirect + callback), Profile "Link VK" control,
  boot callback handling; a merge re-authorizes for a fresh code (VK codes are
  single-use). Web-only (a redirect strands a Mini App webview).
- Deploy: VITE_VK_APP_ID + VITE_VK_ID_REDIRECT_URL build args + gateway env,
  ci.yaml/prod-deploy TEST_/PROD_ vars, compose/Dockerfile/.env.example/README.
- Tests: vkid exchange unit (string/number user_id, id_token fallback, errors),
  transcode link.vk, backend ConfirmVK/MergeVK inttest, codec encodeLinkVK.
- Docs: ARCHITECTURE §4, FUNCTIONAL(+ru), gateway README.

ui/src/lib/mock/client.ts

@@ -674,6 +674,16 @@ export class MockGateway implements GatewayClient {
     this.profile.telegramLinked = true;
     return { ...emptyLinked(), status: 'merged' };
   }
+  async linkVK(_code: string, _deviceId: string, _codeVerifier: string): Promise<LinkResult> {
+    this.profile.isGuest = false;
+    this.profile.vkLinked = true;
+    return emptyLinked();
+  }
+  async linkVKMerge(_code: string, _deviceId: string, _codeVerifier: string): Promise<LinkResult> {
+    this.profile.isGuest = false;
+    this.profile.vkLinked = true;
+    return { ...emptyLinked(), status: 'merged' };
+  }
   async linkUnlink(kind: string): Promise<LinkResult> {
     if (kind === 'telegram') this.profile.telegramLinked = false;
     if (kind === 'vk') this.profile.vkLinked = false;