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>

internal/merger/merger.go

@@ -202,7 +202,7 @@ func (a *AircraftState) MarshalJSON() ([]byte, error) {
 		SpeedHistory:    a.SpeedHistory,
 		Distance:        a.Distance,
 		Bearing:         a.Bearing,
-		Age:             a.Age,
+		Age:             time.Since(a.LastUpdate).Seconds(),
 		MLATSources:     a.MLATSources,
 		PositionSource:  a.PositionSource,
 		UpdateRate:      a.UpdateRate,