fix: Implement dynamic age calculation for aircraft tracking

Resolves issue #3 where aircraft age field was always showing 0 seconds.

Backend Changes:
- Fix MarshalJSON in merger.go to calculate age dynamically using time.Since(a.LastUpdate)
- Add comprehensive tests for age calculation in merger_test.go

Frontend Changes:
- Add calculateAge() utility method to both aircraft-manager.js and ui-manager.js
- Update popup content and aircraft table to use client-side age calculation
- Implement real-time age updates every second in app.js
- Add updateOpenPopupAges() function to refresh popup displays

Key Benefits:
- Aircraft age now shows actual seconds since last transmission
- Real-time updates in UI without requiring new WebSocket data
- Proper data staleness indicators for aviation tracking
- No additional server load - client-side calculation using last_update timestamp

🤖 Generated with [Claude Code](https://claude.ai/code)

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

assets/static/js/app.js

@@ -469,10 +469,40 @@ class SkyView {
         this.radar3d.renderer.render(this.radar3d.scene, this.radar3d.camera);
     }
 
+    updateOpenPopupAges() {
+        // Find any open aircraft popups and update their age displays
+        if (!this.aircraftManager) return;
+        
+        this.aircraftManager.aircraftMarkers.forEach((marker, icao) => {
+            if (marker.isPopupOpen()) {
+                const aircraft = this.aircraftManager.aircraftData.get(icao);
+                if (aircraft) {
+                    // Refresh the popup content with current age
+                    marker.setPopupContent(this.aircraftManager.createPopupContent(aircraft));
+                    
+                    // Re-enhance callsign display for the updated popup
+                    const popupElement = marker.getPopup().getElement();
+                    if (popupElement) {
+                        this.aircraftManager.enhanceCallsignDisplay(popupElement);
+                    }
+                }
+            }
+        });
+    }
+
     startPeriodicTasks() {
         // Update clocks every second
         setInterval(() => this.uiManager.updateClocks(), 1000);
         
+        // Update aircraft ages and refresh displays every second
+        setInterval(() => {
+            // Update aircraft table to show current ages
+            this.uiManager.updateAircraftTable();
+            
+            // Update any open aircraft popups with current ages
+            this.updateOpenPopupAges();
+        }, 1000);
+        
         // Update charts every 10 seconds
         setInterval(() => this.updateCharts(), 10000);