Fix transponder information display and add signal quality foundation

**Transponder Display - WORKING ✅**
- Fixed: TransponderCapability now appears in popup (showing "Enhanced", "Level 2+", etc.)
- Added transponder field handling in merger.go mergeAircraftData()
- Shortened labels: "Level 2+" instead of "Level 2+ Transponder"
- Shows in popup when DF11 All-Call Reply messages are received

**Signal Quality Implementation - IN PROGRESS ⚠️**
- Added SignalQuality field to Aircraft struct and JSON marshaling
- Added calculateSignalQuality() function with quality levels: Excellent/Good/Fair/Poor
- Added signal quality field merging logic with intelligent quality prioritization
- Extended squitter messages set baseline "Good" quality
- Enhanced NACp extraction from airborne position messages (TC 9-18)

**Current Status:**
- ✅ Transponder info displays correctly in popup
- ⚠️ Signal quality implementation complete but not appearing in popup yet
- ⚠️ Needs investigation of data flow between decoder and frontend

**Next Steps:**
- Debug why SignalQuality field remains empty in API responses
- Verify signal quality calculation is being called for received message types
- Test with live ADS-B data to confirm field population

The transponder capability display issue is now resolved. Users can see
transponder levels in aircraft popups when available.

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

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

internal/merger/merger.go

@@ -557,6 +557,27 @@ func (m *Merger) mergeAircraftData(state *AircraftState, new *modes.Aircraft, so
 	if new.BaroSetting != 0 {
 		state.BaroSetting = new.BaroSetting
 	}
+
+	// Transponder information - use most recent non-empty
+	if new.TransponderCapability != "" {
+		state.TransponderCapability = new.TransponderCapability
+	}
+	if new.TransponderLevel > 0 {
+		state.TransponderLevel = new.TransponderLevel
+	}
+	
+	// Signal quality - use most recent non-empty (prefer higher quality assessments)
+	if new.SignalQuality != "" {
+		// Simple quality ordering: Excellent > Good > Fair > Poor
+		shouldUpdate := state.SignalQuality == "" ||
+			(new.SignalQuality == "Excellent") ||
+			(new.SignalQuality == "Good" && state.SignalQuality != "Excellent") ||
+			(new.SignalQuality == "Fair" && state.SignalQuality == "Poor")
+		
+		if shouldUpdate {
+			state.SignalQuality = new.SignalQuality
+		}
+	}
 }
 
 // updateHistories adds data points to historical tracking arrays.