Implement transponder code (squawk) lookup and textual descriptions

Resolves #30

- Add comprehensive squawk code lookup database with emergency, standard, military, and special codes
- Implement squawk.Database with 20+ common transponder codes including:
  * Emergency codes: 7700 (General Emergency), 7600 (Radio Failure), 7500 (Hijacking)
  * Standard codes: 1200/7000 (VFR), operational codes by region
  * Special codes: 1277 (SAR), 1255 (Fire Fighting), military codes
- Add SquawkDescription field to Aircraft struct and JSON marshaling
- Integrate squawk database into merger for automatic description population
- Update frontend with color-coded squawk display and tooltips:
  * Red for emergency codes with warning symbols
  * Orange for special operations
  * Gray for military codes
  * Green for standard operational codes
- Add comprehensive test coverage for squawk lookup functionality

Features:
- Visual emergency code identification for safety
- Educational descriptions for aviation enthusiasts
- Configurable lookup system for regional variations
- Hover tooltips with full code explanations
- Graceful fallback for unknown codes

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

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

internal/merger/merger.go

@@ -29,6 +29,7 @@ import (
 
 	"skyview/internal/icao"
 	"skyview/internal/modes"
+	"skyview/internal/squawk"
 )
 
 const (
@@ -126,6 +127,7 @@ func (a *AircraftState) MarshalJSON() ([]byte, error) {
 		Heading               int     `json:"Heading"`
 		Category              string  `json:"Category"`
 		Squawk                string  `json:"Squawk"`
+		SquawkDescription     string  `json:"SquawkDescription"`
 		Emergency             string  `json:"Emergency"`
 		OnGround              bool    `json:"OnGround"`
 		Alert                 bool    `json:"Alert"`
@@ -173,6 +175,7 @@ func (a *AircraftState) MarshalJSON() ([]byte, error) {
 		Heading:               a.Aircraft.Heading,
 		Category:              a.Aircraft.Category,
 		Squawk:                a.Aircraft.Squawk,
+		SquawkDescription:     a.Aircraft.SquawkDescription,
 		Emergency:             a.Aircraft.Emergency,
 		OnGround:              a.Aircraft.OnGround,
 		Alert:                 a.Aircraft.Alert,
@@ -268,6 +271,7 @@ type Merger struct {
 	aircraft      map[uint32]*AircraftState // ICAO24 -> merged aircraft state
 	sources       map[string]*Source        // Source ID -> source information
 	icaoDB        *icao.Database            // ICAO country lookup database
+	squawkDB      *squawk.Database          // Transponder code lookup database
 	mu            sync.RWMutex              // Protects all maps and slices
 	historyLimit  int                       // Maximum history points to retain
 	staleTimeout  time.Duration             // Time before aircraft considered stale (15 seconds)
@@ -296,10 +300,13 @@ func NewMerger() (*Merger, error) {
 		return nil, fmt.Errorf("failed to initialize ICAO database: %w", err)
 	}
 
+	squawkDB := squawk.NewDatabase()
+
 	return &Merger{
 		aircraft:      make(map[uint32]*AircraftState),
 		sources:       make(map[string]*Source),
 		icaoDB:        icaoDB,
+		squawkDB:      squawkDB,
 		historyLimit:  500,
 		staleTimeout:  15 * time.Second, // Aircraft timeout - reasonable for ADS-B tracking
 		updateMetrics: make(map[uint32]*updateMetric),
@@ -535,6 +542,8 @@ func (m *Merger) mergeAircraftData(state *AircraftState, new *modes.Aircraft, so
 	}
 	if new.Squawk != "" {
 		state.Squawk = new.Squawk
+		// Look up squawk description
+		state.SquawkDescription = m.squawkDB.FormatSquawkWithDescription(new.Squawk)
 	}
 	if new.Category != "" {
 		state.Category = new.Category