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.

gateway/Dockerfile

@@ -25,12 +25,16 @@ ARG VITE_TELEGRAM_BOT_ID=
 ARG VITE_TELEGRAM_LINK=
 ARG VITE_TELEGRAM_GAME_CHANNEL_NAME=
 ARG VITE_VK_APP_LINK=
+ARG VITE_VK_APP_ID=
+ARG VITE_VK_ID_REDIRECT_URL=
 ARG VITE_GATEWAY_URL=
 ARG VITE_APP_VERSION=
 ENV VITE_TELEGRAM_BOT_ID=$VITE_TELEGRAM_BOT_ID \
     VITE_TELEGRAM_LINK=$VITE_TELEGRAM_LINK \
     VITE_TELEGRAM_GAME_CHANNEL_NAME=$VITE_TELEGRAM_GAME_CHANNEL_NAME \
     VITE_VK_APP_LINK=$VITE_VK_APP_LINK \
+    VITE_VK_APP_ID=$VITE_VK_APP_ID \
+    VITE_VK_ID_REDIRECT_URL=$VITE_VK_ID_REDIRECT_URL \
     VITE_GATEWAY_URL=$VITE_GATEWAY_URL \
     VITE_APP_VERSION=$VITE_APP_VERSION