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,

internal/merger/merger_test.go

@@ -0,0 +1,92 @@
+package merger
+
+import (
+	"encoding/json"
+	"testing"
+	"time"
+
+	"skyview/internal/modes"
+)
+
+func TestAircraftState_MarshalJSON_AgeCalculation(t *testing.T) {
+	// Create a test aircraft state with a known LastUpdate time
+	pastTime := time.Now().Add(-30 * time.Second) // 30 seconds ago
+
+	state := &AircraftState{
+		Aircraft: &modes.Aircraft{
+			ICAO24:   0x123456,
+			Callsign: "TEST123",
+		},
+		LastUpdate: pastTime,
+	}
+
+	// Marshal to JSON
+	jsonData, err := json.Marshal(state)
+	if err != nil {
+		t.Fatalf("Failed to marshal AircraftState: %v", err)
+	}
+
+	// Parse the JSON to check the age field
+	var result map[string]interface{}
+	if err := json.Unmarshal(jsonData, &result); err != nil {
+		t.Fatalf("Failed to unmarshal JSON: %v", err)
+	}
+
+	// Check that age is not 0 and is approximately 30 seconds
+	age, ok := result["age"].(float64)
+	if !ok {
+		t.Fatal("Age field not found or not a number")
+	}
+
+	if age < 25 || age > 35 {
+		t.Errorf("Expected age to be around 30 seconds, got %f", age)
+	}
+
+	if age == 0 {
+		t.Error("Age should not be 0 - this indicates the bug is still present")
+	}
+
+	t.Logf("Aircraft age calculated correctly: %f seconds", age)
+}
+
+func TestAircraftState_MarshalJSON_ZeroAge(t *testing.T) {
+	// Test with very recent time to ensure age is very small but not exactly 0
+	recentTime := time.Now()
+
+	state := &AircraftState{
+		Aircraft: &modes.Aircraft{
+			ICAO24:   0x123456,
+			Callsign: "TEST123",
+		},
+		LastUpdate: recentTime,
+	}
+
+	// Marshal to JSON
+	jsonData, err := json.Marshal(state)
+	if err != nil {
+		t.Fatalf("Failed to marshal AircraftState: %v", err)
+	}
+
+	// Parse the JSON to check the age field
+	var result map[string]interface{}
+	if err := json.Unmarshal(jsonData, &result); err != nil {
+		t.Fatalf("Failed to unmarshal JSON: %v", err)
+	}
+
+	// Check that age is very small but should not be exactly 0 (unless extremely fast)
+	age, ok := result["age"].(float64)
+	if !ok {
+		t.Fatal("Age field not found or not a number")
+	}
+
+	if age < 0 {
+		t.Errorf("Age should not be negative, got %f", age)
+	}
+
+	// Age should be very small (less than 1 second) but might not be exactly 0
+	if age > 1.0 {
+		t.Errorf("Age should be very small for recent update, got %f", age)
+	}
+
+	t.Logf("Recent aircraft age calculated correctly: %f seconds", age)
+}