fix: lockout countdown shows NaN minutes

The API returns locked_until already ending in 'Z' (UTC ISO format).
Appending another 'Z' produces an invalid date string, so Date arithmetic
yielded NaN. Remove the redundant suffix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/src/pages/Login.jsx

@@ -23,7 +23,7 @@ export default function Login() {
       if (err.response?.status === 423) {
         const lockedUntil = err.response?.data?.locked_until;
         if (lockedUntil) {
-          const mins = Math.ceil((new Date(lockedUntil + 'Z') - Date.now()) / 60000);
+          const mins = Math.ceil((new Date(lockedUntil) - Date.now()) / 60000);
           setError(`Account locked. Try again in ${mins} minute${mins !== 1 ? 's' : ''}.`);
         } else {
           setError('Account locked. Too many failed attempts. Try again later.');