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.

.gitea/workflows/ci.yaml

@@ -330,6 +330,10 @@ jobs:
           # VK Mini App protected key (offline HMAC for the launch-param signature); empty
           # leaves the VK auth path (auth.vk) disabled until the operator sets the secret.
           GATEWAY_VK_APP_SECRET: ${{ secrets.TEST_GATEWAY_VK_APP_SECRET }}
+          # VK ID web login (browser VK-identity linking): the VK ID "Web" app's protected key
+          # for the server-side confidential code exchange — a SEPARATE VK app from the Mini
+          # App above. Empty leaves the link.vk.* ops disabled.
+          GATEWAY_VK_ID_CLIENT_SECRET: ${{ secrets.TEST_GATEWAY_VK_ID_CLIENT_SECRET }}
           # Signs the finished-game export download URLs (backend + compose interpolation).
           EXPORT_SIGN_KEY: ${{ secrets.TEST_EXPORT_SIGN_KEY }}
           # Transactional email via the shared Selectel relay. Empty host leaves the
@@ -369,6 +373,10 @@ jobs:
           VITE_TELEGRAM_LINK: ${{ vars.TEST_VITE_TELEGRAM_LINK }}
           VITE_TELEGRAM_GAME_CHANNEL_NAME: ${{ vars.TEST_VITE_TELEGRAM_GAME_CHANNEL_NAME }}
           VITE_VK_APP_LINK: ${{ vars.TEST_VITE_VK_APP_LINK }}
+          # VK ID web login: the "Web" app id + its trusted redirect URL. One value each feeds
+          # both the SPA (the authorize URL) and the gateway (client_id / exchange redirect_uri).
+          VITE_VK_APP_ID: ${{ vars.TEST_VITE_VK_APP_ID }}
+          VITE_VK_ID_REDIRECT_URL: ${{ vars.TEST_VK_ID_REDIRECT_URL }}
           VITE_GATEWAY_URL: ${{ vars.TEST_VITE_GATEWAY_URL }}
           # Unset vars render empty -> the compose ":-" defaults apply.
           POSTGRES_DB: ${{ vars.TEST_POSTGRES_DB }}