Fix clock flickering from duplicate setInterval timers

app.js and nav.html both started 1-second intervals updating the same
#headerUtcTime element. Even though both used InterceptTime, their
slightly different timing caused visible text flicker.

Fix: app.js now sets window._navClockStarted before starting its
interval, so nav.html's guard condition skips its duplicate. Also
register InterceptTime.onChange listener in app.js for instant updates.

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

static/js/core/app.js

@@ -465,7 +465,11 @@ function initApp() {
     // Start clock and init time settings
     initTimeSettings();
     updateHeaderClock();
+    window._navClockStarted = true;  // Prevent nav.html from starting a duplicate interval
     setInterval(updateHeaderClock, 1000);
+    if (typeof InterceptTime !== 'undefined' && InterceptTime.onChange) {
+        InterceptTime.onChange(updateHeaderClock);
+    }
 
     // Load bias-T setting
     loadBiasTSetting();