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/merger/merger.go

@@ -795,10 +795,13 @@ func (m *Merger) GetSources() []*Source {
 //
 // The statistics include:
 //   - total_aircraft: Current number of tracked aircraft
+//   - aircraft_with_position: Number of aircraft with valid position data
+//   - aircraft_without_position: Number of aircraft without position data
 //   - total_messages: Sum of all messages processed
 //   - active_sources: Number of currently connected sources
 //   - aircraft_by_sources: Distribution of aircraft by number of tracking sources
 //
+// The position statistics help assess data quality and tracking effectiveness.
 // The aircraft_by_sources map shows data quality - aircraft tracked by
 // multiple sources generally have better position accuracy and reliability.
 //
@@ -810,11 +813,22 @@ func (m *Merger) GetStatistics() map[string]interface{} {
 	totalMessages := int64(0)
 	activeSources := 0
 	aircraftBySources := make(map[int]int) // Count by number of sources
+	aircraftWithPosition := 0
+	aircraftWithoutPosition := 0
 
 	for _, state := range m.aircraft {
 		totalMessages += state.TotalMessages
 		numSources := len(state.Sources)
 		aircraftBySources[numSources]++
+
+		// Check if aircraft has valid position data
+		if state.Aircraft.PositionValid &&
+			state.Aircraft.Latitude != 0.0 &&
+			state.Aircraft.Longitude != 0.0 {
+			aircraftWithPosition++
+		} else {
+			aircraftWithoutPosition++
+		}
 	}
 
 	for _, src := range m.sources {
@@ -824,10 +838,12 @@ func (m *Merger) GetStatistics() map[string]interface{} {
 	}
 
 	return map[string]interface{}{
-		"total_aircraft":      len(m.aircraft),
-		"total_messages":      totalMessages,
-		"active_sources":      activeSources,
-		"aircraft_by_sources": aircraftBySources,
+		"total_aircraft":            len(m.aircraft),
+		"aircraft_with_position":    aircraftWithPosition,
+		"aircraft_without_position": aircraftWithoutPosition,
+		"total_messages":            totalMessages,
+		"active_sources":            activeSources,
+		"aircraft_by_sources":       aircraftBySources,
 	}
 }