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/modules/ui-manager.js

@@ -129,7 +129,7 @@ export class UIManager {
             <td>${aircraft.Track || '-'}°</td>
             <td>${sources}</td>
             <td><span class="${this.getSignalClass(bestSignal)}">${bestSignal ? bestSignal.toFixed(1) : '-'}</span></td>
-            <td>${aircraft.Age ? aircraft.Age.toFixed(0) : '0'}s</td>
+            <td>${this.calculateAge(aircraft).toFixed(0)}s</td>
         `;
         
         row.addEventListener('click', () => {
@@ -161,6 +161,16 @@ export class UIManager {
         return 'commercial';
     }
 
+    calculateAge(aircraft) {
+        if (!aircraft.last_update) return 0;
+        
+        const lastUpdate = new Date(aircraft.last_update);
+        const now = new Date();
+        const ageMs = now.getTime() - lastUpdate.getTime();
+        
+        return Math.max(0, ageMs / 1000); // Return age in seconds, minimum 0
+    }
+
     getBestSignalFromSources(sources) {
         if (!sources) return null;
         let bestSignal = -999;