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/internal/transcode/transcode.go

@@ -14,6 +14,7 @@ import (
 	"scrabble/gateway/internal/backendclient"
 	"scrabble/gateway/internal/connector"
 	"scrabble/gateway/internal/vkauth"
+	"scrabble/gateway/internal/vkid"
 	fb "scrabble/pkg/fbs/scrabblefb"
 )
 
@@ -152,6 +153,15 @@ func WithVKAuth(secret string) Option {
 	}
 }
 
+// WithVKLink registers the VK web-link ops (link.vk.confirm/merge), which complete a
+// browser VK ID authorization via the server-side code exchange. A nil exchanger leaves
+// them unregistered, so the ops are unknown wherever VK ID web login is not configured.
+func WithVKLink(ex VKIDExchanger) Option {
+	return func(r *Registry, backend *backendclient.Client) {
+		registerVKLinkOps(r, backend, ex)
+	}
+}
+
 // Lookup returns the operation for messageType, and whether it is registered.
 func (r *Registry) Lookup(messageType string) (Op, bool) {
 	op, ok := r.ops[messageType]
@@ -172,6 +182,9 @@ func DomainCode(err error) (string, bool) {
 	if errors.Is(err, vkauth.ErrInvalid) {
 		return "invalid_vk_params", true
 	}
+	if errors.Is(err, vkid.ErrInvalid) {
+		return "invalid_vk_id", true
+	}
 	if errors.Is(err, connector.ErrInvalidLoginWidget) {
 		return "invalid_login_widget", true
 	}