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>

index.html

@@ -180,6 +180,13 @@
       <button class="modal-close" onclick="closeSettingsModal()">&#x2715;</button>
     </div>
     <div class="modal-body">
+      <div class="settings-section">
+        <div class="settings-section-title">Display</div>
+        <div class="settings-row">
+          <label class="settings-label" for="tz-select">Timezone</label>
+          <select id="tz-select" class="form-input settings-select"></select>
+        </div>
+      </div>
       <div class="settings-section">
         <div class="settings-section-title">Export</div>
         <p class="settings-desc">Download all instance data as a JSON backup file.</p>