Fix clock flickering on main page — inline updateHeaderClock was using UTC

The updateHeaderClock function in index.html was inlined and still using
raw UTC (toISOString), while nav.html's version used InterceptTime.
Both ran on 1-second intervals updating the same element, causing the
clock to rapidly alternate between ET and UTC.

Fix: Updated the inline version in index.html to use InterceptTime,
matching nav.html. Added _navClockStarted guard and onChange listener
so only one interval runs and timezone changes apply instantly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/index.html

@@ -3816,11 +3816,17 @@
             keywords: []
         };
 
-        // UTC Clock Update
+        // Clock Update (uses global InterceptTime for timezone/format)
         function updateHeaderClock() {
             const now = new Date();
-            const utc = now.toISOString().substring(11, 19);
-            document.getElementById('headerUtcTime').textContent = utc;
+            const el = document.getElementById('headerUtcTime');
+            const label = document.querySelector('.utc-label');
+            if (typeof InterceptTime !== 'undefined') {
+                if (el) el.textContent = InterceptTime.fullTime(now);
+                if (label) label.textContent = InterceptTime.getLabel() || 'LOCAL';
+            } else {
+                if (el) el.textContent = now.toISOString().substring(11, 19);
+            }
         }
 
         function setActiveModeIndicator(label) {
@@ -3855,8 +3861,12 @@
         }
 
         // Update clock every second
+        window._navClockStarted = true;
         setInterval(updateHeaderClock, 1000);
-        updateHeaderClock();  // Initial call
+        updateHeaderClock();
+        if (typeof InterceptTime !== 'undefined' && InterceptTime.onChange) {
+            InterceptTime.onChange(updateHeaderClock);
+        }
         applyKeyboardAccessibility();
 
         // Pager message filter functions