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/vkid/vkid_test.go

@@ -0,0 +1,131 @@
+package vkid
+
+import (
+	"context"
+	"encoding/base64"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+)
+
+// testExchanger builds an Exchanger pointed at a test server.
+func testExchanger(endpoint string) *Exchanger {
+	return &Exchanger{
+		appID:        "app-1",
+		clientSecret: "secret-1",
+		redirectURI:  "https://example.test/app/",
+		endpoint:     endpoint,
+		httpClient:   &http.Client{},
+	}
+}
+
+func TestExchangeSuccessSendsConfidentialPKCE(t *testing.T) {
+	var gotForm url.Values
+	var gotContentType string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotContentType = r.Header.Get("Content-Type")
+		body, _ := io.ReadAll(r.Body)
+		gotForm, _ = url.ParseQuery(string(body))
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = io.WriteString(w, `{"user_id":"12345","access_token":"a","id_token":"h.e.s"}`)
+	}))
+	defer srv.Close()
+
+	id, err := testExchanger(srv.URL).Exchange(context.Background(), "the-code", "dev-9", "verifier-xyz")
+	if err != nil {
+		t.Fatalf("Exchange: %v", err)
+	}
+	if id.ExternalID != "12345" {
+		t.Fatalf("ExternalID = %q, want 12345", id.ExternalID)
+	}
+	if gotContentType != "application/x-www-form-urlencoded" {
+		t.Errorf("Content-Type = %q", gotContentType)
+	}
+	// The confidential exchange must carry the PKCE inputs and the app credentials.
+	want := map[string]string{
+		"grant_type":    "authorization_code",
+		"code":          "the-code",
+		"code_verifier": "verifier-xyz",
+		"device_id":     "dev-9",
+		"client_id":     "app-1",
+		"client_secret": "secret-1",
+		"redirect_uri":  "https://example.test/app/",
+	}
+	for k, v := range want {
+		if gotForm.Get(k) != v {
+			t.Errorf("form[%s] = %q, want %q", k, gotForm.Get(k), v)
+		}
+	}
+}
+
+func TestExchangeAcceptsNumericUserID(t *testing.T) {
+	// VK returns user_id as a bare JSON number in some responses; it must parse too.
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"user_id":1234567890,"access_token":"a"}`)
+	}))
+	defer srv.Close()
+
+	id, err := testExchanger(srv.URL).Exchange(context.Background(), "c", "d", "v")
+	if err != nil {
+		t.Fatalf("Exchange: %v", err)
+	}
+	if id.ExternalID != "1234567890" {
+		t.Fatalf("ExternalID = %q, want 1234567890", id.ExternalID)
+	}
+}
+
+func TestExchangeFallsBackToIDTokenSub(t *testing.T) {
+	sub := "987654321"
+	payload := base64.RawURLEncoding.EncodeToString([]byte(`{"sub":"` + sub + `"}`))
+	idToken := "header." + payload + ".sig"
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"access_token":"a","id_token":"`+idToken+`"}`)
+	}))
+	defer srv.Close()
+
+	id, err := testExchanger(srv.URL).Exchange(context.Background(), "c", "d", "v")
+	if err != nil {
+		t.Fatalf("Exchange: %v", err)
+	}
+	if id.ExternalID != sub {
+		t.Fatalf("ExternalID = %q, want %q (from id_token sub)", id.ExternalID, sub)
+	}
+}
+
+func TestExchangeRejectedIsErrInvalid(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+		_, _ = io.WriteString(w, `{"error":"invalid_grant","error_description":"code expired"}`)
+	}))
+	defer srv.Close()
+
+	_, err := testExchanger(srv.URL).Exchange(context.Background(), "c", "d", "v")
+	if !errors.Is(err, ErrInvalid) {
+		t.Fatalf("err = %v, want ErrInvalid", err)
+	}
+}
+
+func TestExchangeEmptyInputsFailFast(t *testing.T) {
+	// Missing PKCE inputs are rejected without any network call.
+	ex := testExchanger("http://127.0.0.1:0/never")
+	for _, args := range [][3]string{{"", "d", "v"}, {"c", "", "v"}, {"c", "d", ""}} {
+		if _, err := ex.Exchange(context.Background(), args[0], args[1], args[2]); !errors.Is(err, ErrInvalid) {
+			t.Errorf("Exchange%v err = %v, want ErrInvalid", args, err)
+		}
+	}
+}
+
+func TestExchangeTransportErrorIsNotErrInvalid(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(http.ResponseWriter, *http.Request) {}))
+	srv.Close() // closed listener → connection refused
+	_, err := testExchanger(srv.URL).Exchange(context.Background(), "c", "d", "v")
+	if err == nil {
+		t.Fatal("want a transport error")
+	}
+	if errors.Is(err, ErrInvalid) {
+		t.Fatalf("transport error must not be ErrInvalid: %v", err)
+	}
+}