Fix aircraft markers not updating positions in real-time

Root cause: The merger was blocking position updates from the same source
after the first position was established, designed for multi-source scenarios
but preventing single-source position updates.

Changes:
- Refactor JavaScript into modular architecture (WebSocketManager, AircraftManager, MapManager, UIManager)
- Add CPR coordinate validation to prevent invalid latitude/longitude values
- Fix merger to allow position updates from same source for moving aircraft
- Add comprehensive coordinate bounds checking in CPR decoder
- Update HTML to use new modular JavaScript with cache busting
- Add WebSocket debug logging to track data flow

Technical details:
- CPR decoder now validates coordinates within ±90° latitude, ±180° longitude
- Merger allows updates when currentBest == sourceID (same source continuous updates)
- JavaScript modules provide better separation of concerns and debugging
- WebSocket properly transmits updated aircraft coordinates to frontend

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

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

internal/merger/merger.go

@@ -20,6 +20,8 @@
 package merger
 
 import (
+	"encoding/json"
+	"fmt"
 	"math"
 	"sync"
 	"time"
@@ -72,6 +74,94 @@ type AircraftState struct {
 	UpdateRate      float64                `json:"update_rate"`      // Recent updates per second
 }
 
+// MarshalJSON provides custom JSON marshaling for AircraftState to format ICAO24 as hex.
+func (a *AircraftState) MarshalJSON() ([]byte, error) {
+	// Create a struct that mirrors AircraftState but with ICAO24 as string
+	return json.Marshal(&struct {
+		// From embedded modes.Aircraft
+		ICAO24           string  `json:"ICAO24"`
+		Callsign         string  `json:"Callsign"`
+		Latitude         float64 `json:"Latitude"`
+		Longitude        float64 `json:"Longitude"`
+		Altitude         int     `json:"Altitude"`
+		BaroAltitude     int     `json:"BaroAltitude"`
+		GeomAltitude     int     `json:"GeomAltitude"`
+		VerticalRate     int     `json:"VerticalRate"`
+		GroundSpeed      int     `json:"GroundSpeed"`
+		Track            int     `json:"Track"`
+		Heading          int     `json:"Heading"`
+		Category         string  `json:"Category"`
+		Squawk           string  `json:"Squawk"`
+		Emergency        string  `json:"Emergency"`
+		OnGround         bool    `json:"OnGround"`
+		Alert            bool    `json:"Alert"`
+		SPI              bool    `json:"SPI"`
+		NACp             uint8   `json:"NACp"`
+		NACv             uint8   `json:"NACv"`
+		SIL              uint8   `json:"SIL"`
+		SelectedAltitude int     `json:"SelectedAltitude"`
+		SelectedHeading  float64 `json:"SelectedHeading"`
+		BaroSetting      float64 `json:"BaroSetting"`
+		
+		// From AircraftState
+		Sources         map[string]*SourceData `json:"sources"`
+		LastUpdate      time.Time              `json:"last_update"`
+		FirstSeen       time.Time              `json:"first_seen"`
+		TotalMessages   int64                  `json:"total_messages"`
+		PositionHistory []PositionPoint        `json:"position_history"`
+		SignalHistory   []SignalPoint          `json:"signal_history"`
+		AltitudeHistory []AltitudePoint        `json:"altitude_history"`
+		SpeedHistory    []SpeedPoint           `json:"speed_history"`
+		Distance        float64                `json:"distance"`
+		Bearing         float64                `json:"bearing"`
+		Age             float64                `json:"age"`
+		MLATSources     []string               `json:"mlat_sources"`
+		PositionSource  string                 `json:"position_source"`
+		UpdateRate      float64                `json:"update_rate"`
+	}{
+		// Copy all fields from Aircraft
+		ICAO24:           fmt.Sprintf("%06X", a.Aircraft.ICAO24),
+		Callsign:         a.Aircraft.Callsign,
+		Latitude:         a.Aircraft.Latitude,
+		Longitude:        a.Aircraft.Longitude,
+		Altitude:         a.Aircraft.Altitude,
+		BaroAltitude:     a.Aircraft.BaroAltitude,
+		GeomAltitude:     a.Aircraft.GeomAltitude,
+		VerticalRate:     a.Aircraft.VerticalRate,
+		GroundSpeed:      a.Aircraft.GroundSpeed,
+		Track:            a.Aircraft.Track,
+		Heading:          a.Aircraft.Heading,
+		Category:         a.Aircraft.Category,
+		Squawk:           a.Aircraft.Squawk,
+		Emergency:        a.Aircraft.Emergency,
+		OnGround:         a.Aircraft.OnGround,
+		Alert:            a.Aircraft.Alert,
+		SPI:              a.Aircraft.SPI,
+		NACp:             a.Aircraft.NACp,
+		NACv:             a.Aircraft.NACv,
+		SIL:              a.Aircraft.SIL,
+		SelectedAltitude: a.Aircraft.SelectedAltitude,
+		SelectedHeading:  a.Aircraft.SelectedHeading,
+		BaroSetting:      a.Aircraft.BaroSetting,
+		
+		// Copy all fields from AircraftState
+		Sources:         a.Sources,
+		LastUpdate:      a.LastUpdate,
+		FirstSeen:       a.FirstSeen,
+		TotalMessages:   a.TotalMessages,
+		PositionHistory: a.PositionHistory,
+		SignalHistory:   a.SignalHistory,
+		AltitudeHistory: a.AltitudeHistory,
+		SpeedHistory:    a.SpeedHistory,
+		Distance:        a.Distance,
+		Bearing:         a.Bearing,
+		Age:             a.Age,
+		MLATSources:     a.MLATSources,
+		PositionSource:  a.PositionSource,
+		UpdateRate:      a.UpdateRate,
+	})
+}
+
 // SourceData represents data quality and statistics for a specific source-aircraft pair.
 // This information is used for data fusion decisions and signal quality analysis.
 type SourceData struct {
@@ -133,7 +223,7 @@ type Merger struct {
 	sources       map[string]*Source        // Source ID -> source information
 	mu            sync.RWMutex              // Protects all maps and slices
 	historyLimit  int                      // Maximum history points to retain
-	staleTimeout  time.Duration             // Time before aircraft considered stale
+	staleTimeout  time.Duration             // Time before aircraft considered stale (15 seconds)
 	updateMetrics map[uint32]*updateMetric  // ICAO24 -> update rate calculation data
 }
 
@@ -147,7 +237,7 @@ type updateMetric struct {
 //
 // Default settings:
 //   - History limit: 500 points per aircraft
-//   - Stale timeout: 60 seconds
+//   - Stale timeout: 15 seconds
 //   - Empty aircraft and source maps
 //   - Update metrics tracking enabled
 //
@@ -157,7 +247,7 @@ func NewMerger() *Merger {
 		aircraft:      make(map[uint32]*AircraftState),
 		sources:       make(map[string]*Source),
 		historyLimit:  500,
-		staleTimeout:  60 * time.Second,
+		staleTimeout:  15 * time.Second, // Aircraft timeout - reasonable for ADS-B tracking
 		updateMetrics: make(map[uint32]*updateMetric),
 	}
 }
@@ -219,6 +309,8 @@ func (m *Merger) UpdateAircraft(sourceID string, aircraft *modes.Aircraft, signa
 			updates: make([]time.Time, 0),
 		}
 	}
+	// Note: For existing aircraft, we don't overwrite state.Aircraft here
+	// The mergeAircraftData function will handle selective field updates
 
 	// Update or create source data
 	srcData, srcExists := state.Sources[sourceID]
@@ -294,12 +386,16 @@ func (m *Merger) mergeAircraftData(state *AircraftState, new *modes.Aircraft, so
 		updatePosition := false
 
 		if state.Latitude == 0 {
+			// First position update
 			updatePosition = true
 		} else if srcData, ok := state.Sources[sourceID]; ok {
 			// Use position from source with strongest signal
 			currentBest := m.getBestSignalSource(state)
 			if currentBest == "" || srcData.SignalLevel > state.Sources[currentBest].SignalLevel {
 				updatePosition = true
+			} else if currentBest == sourceID {
+				// Same source as current best - allow updates for moving aircraft
+				updatePosition = true
 			}
 		}
 
@@ -615,7 +711,7 @@ func (m *Merger) GetStatistics() map[string]interface{} {
 // CleanupStale removes aircraft that haven't been updated recently.
 //
 // Aircraft are considered stale if they haven't received updates for longer
-// than staleTimeout (default 60 seconds). This cleanup prevents memory
+// than staleTimeout (default 15 seconds). This cleanup prevents memory
 // growth from aircraft that have left the coverage area or stopped transmitting.
 //
 // The cleanup also removes associated update metrics to free memory.