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/client.ts

@@ -177,6 +177,10 @@ export interface GatewayClient {
   linkEmailMerge(email: string, code: string): Promise<LinkResult>;
   linkTelegram(data: string): Promise<LinkResult>;
   linkTelegramMerge(data: string): Promise<LinkResult>;
+  /** Link a VK identity from the web: the args are the PKCE outputs of the VK ID
+   *  authorization callback, exchanged for the trusted vk id on the gateway. */
+  linkVK(code: string, deviceId: string, codeVerifier: string): Promise<LinkResult>;
+  linkVKMerge(code: string, deviceId: string, codeVerifier: string): Promise<LinkResult>;
   /** Detach a platform identity (kind 'telegram' | 'vk') from the account; email is never
    *  unlinked. Returns the refreshed link result (status 'unlinked'). */
   linkUnlink(kind: string): Promise<LinkResult>;