fix: test connection reads form values instead of saved config

The "Kapcsolat tesztelése" button sent an empty POST to /settings/test,
which read from the (possibly empty) config file on disk. Now sends the
current form values so testing works before clicking Save.

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

app/templates/settings.html

@@ -39,7 +39,8 @@ async function testConnection() {
     result.innerHTML = '<span class="spinner"></span>';
 
     try {
-        const resp = await fetch('/settings/test', { method: 'POST' });
+        const form = new FormData(document.querySelector('form'));
+        const resp = await fetch('/settings/test', { method: 'POST', body: form });
         const data = await resp.json();
         if (data.ok) {
             const v = data.data.version || '?';