Fix issue #21 and add aircraft position tracking indicators

- Fix Debian package upgrade issue by separating upgrade vs remove behavior in prerm script
- Add aircraft position tracking statistics in merger GetStatistics() method
- Update frontend to display position tracking indicators in both header and stats view
- Format Go code to maintain consistency

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

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

internal/modes/decoder.go

@@ -157,10 +157,10 @@ type Aircraft struct {
 	BaroSetting      float64 // Barometric pressure setting (QNH) in millibars
 
 	// Additional fields from VRS JSON and extended sources
-	Registration      string // Aircraft registration (e.g., "N12345")
-	AircraftType      string // Aircraft type (e.g., "B738")
-	Operator          string // Airline or operator name
-	
+	Registration string // Aircraft registration (e.g., "N12345")
+	AircraftType string // Aircraft type (e.g., "B738")
+	Operator     string // Airline or operator name
+
 	// Validity flags for optional fields (used by VRS and other sources)
 	CallsignValid     bool // Whether callsign is valid
 	PositionValid     bool // Whether position is valid