feat: Enhance flight trails with historical altitude data and fix label display

- Extend PositionPoint struct to include altitude field in position history
- Update position history population to store altitude with each position
- Fix 3D aircraft labels to show GroundSpeed instead of undefined Speed field
- Enable true 3D flight trails with proper altitude visualization
- Improve trail accuracy by using historical altitude data per position

Resolves trails showing flat paths and labels showing N/A for speed.

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

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

internal/merger/merger.go

@@ -227,10 +227,11 @@ type SourceData struct {
 // PositionPoint represents a timestamped position update in aircraft history.
 // Used to build position trails for visualization and track analysis.
 type PositionPoint struct {
-	Time      time.Time `json:"time"`   // Timestamp when position was received
-	Latitude  float64   `json:"lat"`    // Latitude in decimal degrees
-	Longitude float64   `json:"lon"`    // Longitude in decimal degrees
-	Source    string    `json:"source"` // Source that provided this position
+	Time      time.Time `json:"time"`     // Timestamp when position was received
+	Latitude  float64   `json:"lat"`      // Latitude in decimal degrees
+	Longitude float64   `json:"lon"`      // Longitude in decimal degrees
+	Altitude  int       `json:"altitude"` // Altitude in feet (0 if unknown)
+	Source    string    `json:"source"`   // Source that provided this position
 }
 
 // SignalPoint represents a timestamped signal strength measurement.
@@ -655,6 +656,7 @@ func (m *Merger) updateHistories(state *AircraftState, aircraft *modes.Aircraft,
 				Time:      timestamp,
 				Latitude:  aircraft.Latitude,
 				Longitude: aircraft.Longitude,
+				Altitude:  aircraft.Altitude, // Include altitude in position history
 				Source:    sourceID,
 			})
 		}