refactor: const/let modernisation and eliminate duplicate date-parse logic

- Replace all var declarations in base.html, index.html scripts with
  const/let (const for bindings that are never reassigned, let otherwise)
- Add _toIso() helper to links.html script block and replace the two
  inline .replace(' UTC','Z').replace(' ','T') patterns with it
- Replace var with const in links.html _linksInterval

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

templates/index.html

@@ -466,7 +466,7 @@
 {% block scripts %}
 <script>
   // Start auto-refresh using saved settings interval (default 30 s)
-  var _savedInterval = (window.gandalfSettings && window.gandalfSettings.refreshInterval) || 30;
+  const _savedInterval = (window.gandalfSettings && window.gandalfSettings.refreshInterval) || 30;
   if (_savedInterval > 0) lt.autoRefresh.start(refreshAll, _savedInterval * 1000);
 
   // When settings change, restart auto-refresh with new interval
@@ -477,9 +477,9 @@
 
   // ── Topology collapse toggle ───────────────────────────────────
   (function() {
-    var LS_KEY = 'gandalf_topo_collapsed';
-    var btn    = document.getElementById('topo-toggle-btn');
-    var wrap   = document.getElementById('topo-collapsible-wrap');
+    const LS_KEY = 'gandalf_topo_collapsed';
+    const btn    = document.getElementById('topo-toggle-btn');
+    const wrap   = document.getElementById('topo-collapsible-wrap');
     if (!btn || !wrap) return;
 
     function setCollapsed(v) {
@@ -489,7 +489,7 @@
       try { localStorage.setItem(LS_KEY, v ? '1' : '0'); } catch(_) {}
     }
 
-    var saved = false;
+    let saved = false;
     try { saved = localStorage.getItem(LS_KEY) === '1'; } catch(_) {}
     setCollapsed(saved);