Fix auto-refresh ignoring 'Off' setting on page load

Using || 30 / || 60 as a fallback treats refreshInterval=0 (Off) as
falsy and replaces it with the default, causing auto-refresh to start
even when the user saved 'Off'. Replace with nullish coalescing (??)
so only null/undefined triggers the default.

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

templates/index.html

@@ -469,7 +469,7 @@
 {% block scripts %}
 <script>
   // Start auto-refresh using saved settings interval (default 30 s)
-  const _savedInterval = (window.gandalfSettings && window.gandalfSettings.refreshInterval) || 30;
+  const _savedInterval = window.gandalfSettings?.refreshInterval ?? 30;
   if (_savedInterval > 0) lt.autoRefresh.start(refreshAll, _savedInterval * 1000);
 
   // When settings change, restart auto-refresh with new interval

templates/links.html

@@ -571,7 +571,7 @@ async function loadLinks() {
 }
 
 loadLinks();
-const _linksInterval = (window.gandalfSettings && window.gandalfSettings.refreshInterval) || 60;
+const _linksInterval = window.gandalfSettings?.refreshInterval ?? 60;
 if (_linksInterval > 0) lt.autoRefresh.start(loadLinks, Math.max(_linksInterval, 15) * 1000);
 
 window.onGandalfSettingsChanged = function(s) {