feat: timezone setting — display timestamps in selected local timezone

Add a Display section to the settings modal with a timezone dropdown.
Selection is persisted to localStorage and applied to all timestamps via
fmtDate (date-only) and fmtDateFull (date + time + TZ abbreviation, e.g.
"Mar 28, 2026, 2:48 PM EDT"). Changing the timezone live-re-renders the
current page. Defaults to UTC.

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

js/ui.js

@@ -3,6 +3,34 @@ let editingVmid = null;
 let currentVmid = null;
 let toastTimer  = null;
 
+// ── Timezone ──────────────────────────────────────────────────────────────────
+
+const TIMEZONES = [
+  { label: 'UTC',                      tz: 'UTC' },
+  { label: 'Hawaii (HST)',             tz: 'Pacific/Honolulu' },
+  { label: 'Alaska (AKT)',             tz: 'America/Anchorage' },
+  { label: 'Pacific (PT)',             tz: 'America/Los_Angeles' },
+  { label: 'Mountain (MT)',            tz: 'America/Denver' },
+  { label: 'Central (CT)',             tz: 'America/Chicago' },
+  { label: 'Eastern (ET)',             tz: 'America/New_York' },
+  { label: 'Atlantic (AT)',            tz: 'America/Halifax' },
+  { label: 'London (GMT/BST)',         tz: 'Europe/London' },
+  { label: 'Paris / Berlin (CET)',     tz: 'Europe/Paris' },
+  { label: 'Helsinki (EET)',           tz: 'Europe/Helsinki' },
+  { label: 'Istanbul (TRT)',           tz: 'Europe/Istanbul' },
+  { label: 'Dubai (GST)',              tz: 'Asia/Dubai' },
+  { label: 'India (IST)',              tz: 'Asia/Kolkata' },
+  { label: 'Singapore (SGT)',          tz: 'Asia/Singapore' },
+  { label: 'China (CST)',              tz: 'Asia/Shanghai' },
+  { label: 'Japan / Korea (JST/KST)', tz: 'Asia/Tokyo' },
+  { label: 'Sydney (AEST)',            tz: 'Australia/Sydney' },
+  { label: 'Auckland (NZST)',          tz: 'Pacific/Auckland' },
+];
+
+function getTimezone() {
+  return localStorage.getItem('catalyst_tz') || 'UTC';
+}
+
 // ── Helpers ───────────────────────────────────────────────────────────────────
 
 function esc(str) {
@@ -14,14 +42,14 @@ function esc(str) {
 function fmtDate(d) {
   if (!d) return '—';
   try {
-    return new Date(d).toLocaleDateString('en-US', { year: 'numeric', month: 'short', day: 'numeric' });
+    return new Date(d).toLocaleDateString('en-US', { year: 'numeric', month: 'short', day: 'numeric', timeZone: getTimezone() });
   } catch (e) { return d; }
 }
 
 function fmtDateFull(d) {
   if (!d) return '—';
   try {
-    return new Date(d).toLocaleString('en-US', { year: 'numeric', month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit' });
+    return new Date(d).toLocaleString('en-US', { year: 'numeric', month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', timeZone: getTimezone(), timeZoneName: 'short' });
   } catch (e) { return d; }
 }
 
@@ -289,6 +317,16 @@ function showToast(msg, type = 'success') {
 // ── Settings Modal ────────────────────────────────────────────────────────────
 
 function openSettingsModal() {
+  const sel = document.getElementById('tz-select');
+  if (!sel.options.length) {
+    for (const { label, tz } of TIMEZONES) {
+      const opt = document.createElement('option');
+      opt.value = tz;
+      opt.textContent = label;
+      sel.appendChild(opt);
+    }
+  }
+  sel.value = getTimezone();
   document.getElementById('settings-modal').classList.add('open');
 }
 
@@ -353,3 +391,10 @@ document.getElementById('confirm-overlay').addEventListener('click', e => {
 document.getElementById('settings-modal').addEventListener('click', e => {
   if (e.target === document.getElementById('settings-modal')) closeSettingsModal();
 });
+
+document.getElementById('tz-select').addEventListener('change', e => {
+  localStorage.setItem('catalyst_tz', e.target.value);
+  const m = window.location.pathname.match(/^\/instance\/(\d+)/);
+  if (m) renderDetailPage(parseInt(m[1], 10));
+  else renderDashboard();
+});