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.

deploy/.env.example

@@ -68,6 +68,8 @@ VITE_TELEGRAM_BOT_ID=
 VITE_TELEGRAM_LINK=                                 # friend-invite Mini App link (full URL, e.g. https://t.me/<bot>/<app>)
 VITE_TELEGRAM_GAME_CHANNEL_NAME=                    # landing "Play in Telegram" link, the bot's game channel
 VITE_VK_APP_LINK=                                   # landing "Play on VK" link (full URL, https://vk.com/app<id>)
+VITE_VK_APP_ID=                                     # VK ID "Web" app id (client_id) for VK web-login linking; also the gateway's GATEWAY_VK_ID_APP_ID
+VITE_VK_ID_REDIRECT_URL=                            # VK ID trusted redirect URL (e.g. https://erudit-game.ru/app/); also the gateway's exchange redirect_uri
 VITE_GATEWAY_URL=
 
 # --- Grafana ----------------------------------------------------------------
@@ -97,3 +99,7 @@ TELEGRAM_API_BASE_URL=
 # launch-parameter signature in-process under it (a pure offline HMAC, no VK API call).
 # Empty disables the VK auth path (auth.vk). Set from the Gitea TEST_/PROD_ secret.
 GATEWAY_VK_APP_SECRET=
+# VK ID web login (browser VK-identity linking) — a SEPARATE VK "Web" app: the gateway runs
+# the confidential OAuth 2.1 code exchange under this protected key. The app id + redirect URL
+# come from VITE_VK_APP_ID / VITE_VK_ID_REDIRECT_URL above. All three empty disables link.vk.*.
+GATEWAY_VK_ID_CLIENT_SECRET=