fix(panel): share TOTP skew tolerance with VerifyTwoFactorCode

Move the +/-1 window helper to internal/util/totp so both 2FA
acceptance points use it: login (CheckUser) and disable/rebind plus
username/password changes (VerifyTwoFactorCode). Also shrink comments
to the 2-line house rule and anchor the unit test mid-window to avoid
a step-boundary flake.

Addresses review on #6546 (MEDIUM + 2 LOWs).

internal/util/totp/totp.go

@@ -0,0 +1,22 @@
+package totp
+
+import (
+	"time"
+
+	"github.com/xlzd/gotp"
+)
+
+// SkewWindows is how many 30s steps around now VerifyWithSkew accepts.
+// Standard TOTP clock-drift tolerance, see MHSanaei/3x-ui#6535.
+const SkewWindows = 1
+
+// VerifyWithSkew accepts the code for the current step plus/minus SkewWindows.
+func VerifyWithSkew(secret, code string, now time.Time) bool {
+	totp := gotp.NewDefaultTOTP(secret)
+	for i := -SkewWindows; i <= SkewWindows; i++ {
+		if totp.AtTime(now.Add(time.Duration(i*30)*time.Second)) == code {
+			return true
+		}
+	}
+	return false
+}

internal/util/totp/totp_test.go

@@ -0,0 +1,34 @@
+package totp
+
+import (
+	"testing"
+	"time"
+
+	"github.com/xlzd/gotp"
+)
+
+func TestVerifyWithSkew(t *testing.T) {
+	secret := "JBSWY3DPEHPK3PXP"
+	totp := gotp.NewDefaultTOTP(secret)
+	// Anchor mid-window so a step boundary can't fall between sampling and verify.
+	now := time.Unix((time.Now().Unix()/30)*30+15, 0).UTC()
+
+	if !VerifyWithSkew(secret, totp.AtTime(now), now) {
+		t.Fatal("current window code should verify")
+	}
+	if !VerifyWithSkew(secret, totp.AtTime(now.Add(-30*time.Second)), now) {
+		t.Fatal("previous window code should verify (clock skew)")
+	}
+	if !VerifyWithSkew(secret, totp.AtTime(now.Add(30*time.Second)), now) {
+		t.Fatal("next window code should verify (clock skew)")
+	}
+	if VerifyWithSkew(secret, totp.AtTime(now.Add(-60*time.Second)), now) {
+		t.Fatal("code two windows old should not verify")
+	}
+	if VerifyWithSkew(secret, totp.AtTime(now.Add(60*time.Second)), now) {
+		t.Fatal("code two windows ahead should not verify")
+	}
+	if VerifyWithSkew(secret, "000000", now) {
+		t.Fatal("wrong code should not verify")
+	}
+}