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/base.html

@@ -324,7 +324,7 @@
       lt.init({ bootName: 'GANDALF' });
 
       // Theme toggle
-      var themeBtn = document.getElementById('lt-theme-btn');
+      const themeBtn = document.getElementById('lt-theme-btn');
       if (themeBtn) themeBtn.addEventListener('click', function() { lt.theme.toggle(); });
 
       // Command palette
@@ -343,9 +343,9 @@
 
     // ── Global footer + key actions ───────────────────────────────────────
     document.addEventListener('click', function(e) {
-      var btn = e.target.closest('[data-action]');
+      const btn = e.target.closest('[data-action]');
       if (!btn) return;
-      var action = btn.getAttribute('data-action');
+      const action = btn.getAttribute('data-action');
       if (action === 'show-keyboard-help' && window.lt)  lt.modal.open('lt-keys-help');
       if (action === 'open-settings' && window.lt)       lt.modal.open('lt-settings-modal');
     });
@@ -366,8 +366,8 @@
 
     // ── Settings modal ────────────────────────────────────────────────────
     (function() {
-      var LS_KEY = 'gandalf_settings';
-      var DEFAULT = { refreshInterval: 30 };
+      const LS_KEY = 'gandalf_settings';
+      const DEFAULT = { refreshInterval: 30 };
 
       function loadSettings() {
         try { return Object.assign({}, DEFAULT, JSON.parse(localStorage.getItem(LS_KEY) || '{}')); }
@@ -381,11 +381,11 @@
 
       function applyRefreshPillUI(interval) {
         document.querySelectorAll('#settings-refresh-pills .pill').forEach(function(p) {
-          var isActive = parseInt(p.dataset.refreshInterval) === interval;
+          const isActive = parseInt(p.dataset.refreshInterval) === interval;
           p.classList.toggle('active', isActive);
           p.setAttribute('aria-pressed', isActive ? 'true' : 'false');
         });
-        var hint = document.getElementById('settings-refresh-hint');
+        const hint = document.getElementById('settings-refresh-hint');
         if (hint) {
           if (interval === 0)        hint.textContent = 'Auto-refresh disabled.';
           else if (interval < 60)    hint.textContent = 'Refreshes every ' + interval + ' seconds.';
@@ -394,16 +394,16 @@
       }
 
       // Init pill UI from saved settings
-      var _settings = loadSettings();
+      const _settings = loadSettings();
       applyRefreshPillUI(_settings.refreshInterval);
 
       // Expose for pages that need to read it (e.g. index.html for autoRefresh)
       window.gandalfSettings = _settings;
 
       document.addEventListener('click', function(e) {
-        var pill = e.target.closest('#settings-refresh-pills .pill[data-refresh-interval]');
+        const pill = e.target.closest('#settings-refresh-pills .pill[data-refresh-interval]');
         if (!pill) return;
-        var interval = parseInt(pill.dataset.refreshInterval);
+        const interval = parseInt(pill.dataset.refreshInterval);
         _settings.refreshInterval = interval;
         saveSettings(_settings);
         applyRefreshPillUI(interval);
@@ -412,16 +412,16 @@
 
     // ── Notification Bell — shows active monitoring alerts ────────────────
     (function() {
-      var bell     = document.getElementById('lt-notif-bell');
-      var panel    = document.getElementById('lt-notif-panel');
-      var list     = document.getElementById('lt-notif-list');
-      var clearBtn = document.getElementById('lt-notif-clear-btn');
-      var wrapEl   = document.getElementById('lt-notif-wrap');
+      const bell     = document.getElementById('lt-notif-bell');
+      const panel    = document.getElementById('lt-notif-panel');
+      const list     = document.getElementById('lt-notif-list');
+      const clearBtn = document.getElementById('lt-notif-clear-btn');
+      const wrapEl   = document.getElementById('lt-notif-wrap');
       if (!bell || !panel) return;
 
-      var _open      = false;
-      var _lastEvents = [];
-      var LS_READ_KEY = 'gandalf_notif_read_before';
+      let _open       = false;
+      let _lastEvents = [];
+      const LS_READ_KEY = 'gandalf_notif_read_before';
 
       function getReadBefore() {
         try { return parseInt(localStorage.getItem(LS_READ_KEY) || '0'); } catch(_) { return 0; }
@@ -440,20 +440,20 @@
       }
 
       function fmtAgo(dateStr) {
-        var diff = Math.floor((Date.now() - toMs(dateStr)) / 1000);
+        const diff = Math.floor((Date.now() - toMs(dateStr)) / 1000);
         if (diff < 60)    return diff + 's ago';
         if (diff < 3600)  return Math.floor(diff/60) + 'm ago';
         if (diff < 86400) return Math.floor(diff/3600) + 'h ago';
         return Math.floor(diff/86400) + 'd ago';
       }
 
-      var SEV_DOT = { critical: 'var(--red)', warning: 'var(--amber)' };
+      const SEV_DOT = { critical: 'var(--red)', warning: 'var(--amber)' };
 
       function renderAlerts(events) {
         _lastEvents = events || [];
-        var readBefore = getReadBefore();
-        var active = _lastEvents.filter(function(e) { return e.severity !== 'info'; });
-        var unreadCount = active.filter(function(e) { return toMs(e.last_seen) > readBefore; }).length;
+        const readBefore = getReadBefore();
+        const active = _lastEvents.filter(function(e) { return e.severity !== 'info'; });
+        const unreadCount = active.filter(function(e) { return toMs(e.last_seen) > readBefore; }).length;
         lt.notif.set(bell, unreadCount);
 
         if (!active.length) {
@@ -461,8 +461,8 @@
           return;
         }
         list.innerHTML = active.slice(0, 25).map(function(e) {
-          var isUnread = toMs(e.last_seen) > readBefore;
-          var dotColor = SEV_DOT[e.severity] || 'var(--text-muted)';
+          const isUnread = toMs(e.last_seen) > readBefore;
+          const dotColor = SEV_DOT[e.severity] || 'var(--text-muted)';
           return '<div class="lt-notif-item' + (isUnread ? ' lt-notif-item--unread' : '') + '">' +
             '<div class="lt-notif-dot' + (isUnread ? '' : ' lt-notif-dot--read') + '" style="background:' + dotColor + '"></div>' +
             '<div class="lt-notif-item-body">' +
@@ -476,14 +476,14 @@
         fetch('/api/status', { credentials: 'same-origin' })
           .then(function(r) { return r.json(); })
           .then(function(data) {
-            var events = data.events || [];
+            const events = data.events || [];
             if (andRender) {
               renderAlerts(events);
             } else {
               _lastEvents = events;
-              var readBefore = getReadBefore();
-              var active = events.filter(function(e) { return e.severity !== 'info'; });
-              var unread = active.filter(function(e) { return toMs(e.last_seen) > readBefore; }).length;
+              const readBefore = getReadBefore();
+              const active = events.filter(function(e) { return e.severity !== 'info'; });
+              const unread = active.filter(function(e) { return toMs(e.last_seen) > readBefore; }).length;
               lt.notif.set(bell, unread);
             }
           })