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/links.html

@@ -36,6 +36,7 @@
 {% block scripts %}
 <script>
 const escHtml = s => lt.escHtml(s);
+const _toIso  = s => s ? s.replace(' UTC', 'Z').replace(' ', 'T') : s;
 
 // ── Formatting helpers ────────────────────────────────────────────
 function fmtRate(bytesPerSec) {
@@ -326,7 +327,7 @@ function renderPortCard(portName, d) {
 function renderUnifiSwitches(unifiSwitches, dataUpdated) {
   if (!unifiSwitches || !Object.keys(unifiSwitches).length) return '';
   const updStr = dataUpdated
-    ? new Date(dataUpdated.replace(' UTC', 'Z').replace(' ', 'T')).toLocaleTimeString()
+    ? new Date(_toIso(dataUpdated)).toLocaleTimeString()
     : '';
   const html = Object.entries(unifiSwitches).map(([swName, sw]) => {
     const ports = sw.ports || {};
@@ -558,7 +559,7 @@ async function loadLinks() {
 }
 
 loadLinks();
-var _linksInterval = (window.gandalfSettings && window.gandalfSettings.refreshInterval) || 60;
+const _linksInterval = (window.gandalfSettings && window.gandalfSettings.refreshInterval) || 60;
 if (_linksInterval > 0) lt.autoRefresh.start(loadLinks, Math.max(_linksInterval, 15) * 1000);
 
 window.onGandalfSettingsChanged = function(s) {