fix(panel): accept 2FA codes from adjacent TOTP windows

CheckUser compared only gotp.Now(), so a code submitted at the end of
its 30s window (or with slight client/server clock drift) failed with
'invalid 2fa code', while the immediate retry in the next window
succeeded. Accept current +/-1 window, the standard TOTP skew
tolerance.

Fixes MHSanaei/3x-ui#6535

internal/web/service/panel/user_totp_test.go

@@ -0,0 +1,33 @@
+package panel
+
+import (
+	"testing"
+	"time"
+
+	"github.com/xlzd/gotp"
+)
+
+func TestVerifyTOTPWithSkew(t *testing.T) {
+	secret := "JBSWY3DPEHPK3PXP"
+	totp := gotp.NewDefaultTOTP(secret)
+	now := time.Now()
+
+	if !verifyTOTPWithSkew(secret, totp.AtTime(now)) {
+		t.Fatal("current window code should verify")
+	}
+	if !verifyTOTPWithSkew(secret, totp.AtTime(now.Add(-30*time.Second))) {
+		t.Fatal("previous window code should verify (clock skew)")
+	}
+	if !verifyTOTPWithSkew(secret, totp.AtTime(now.Add(30*time.Second))) {
+		t.Fatal("next window code should verify (clock skew)")
+	}
+	if verifyTOTPWithSkew(secret, totp.AtTime(now.Add(-60*time.Second))) {
+		t.Fatal("code two windows old should not verify")
+	}
+	if verifyTOTPWithSkew(secret, totp.AtTime(now.Add(60*time.Second))) {
+		t.Fatal("code two windows ahead should not verify")
+	}
+	if verifyTOTPWithSkew(secret, "000000") {
+		t.Fatal("wrong code should not verify")
+	}
+}