Implement comprehensive ICAO country lookup with complete global coverage

- Fix country field marshaling in JSON output to display countries in web frontend
- Replace incomplete ICAO database with comprehensive allocation table from aerotransport.org
- Add all 120+ countries and territories with correct hex address ranges
- Fix aircraft legend label: change "Medium 34000-136000kg" to "Large 34000-136000kg"
- Ensure complete coverage for all allocated ICAO 24-bit addresses worldwide

Fixes:
- 3C64AE now correctly shows Germany 🇩🇪 (range 3C0000-3FFFFF)
- 4ACB58 now correctly shows Sweden 🇸🇪 (range 4A8000-4AFFFF)
- 04008D now correctly shows Ethiopia 🇪🇹 (range 040000-040FFF)

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

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

internal/merger/merger.go

@@ -122,6 +122,9 @@ func (a *AircraftState) MarshalJSON() ([]byte, error) {
 		MLATSources     []string               `json:"mlat_sources"`
 		PositionSource  string                 `json:"position_source"`
 		UpdateRate      float64                `json:"update_rate"`
+		Country         string                 `json:"country"`
+		CountryCode     string                 `json:"country_code"`
+		Flag            string                 `json:"flag"`
 	}{
 		// Copy all fields from Aircraft
 		ICAO24:           fmt.Sprintf("%06X", a.Aircraft.ICAO24),
@@ -163,6 +166,9 @@ func (a *AircraftState) MarshalJSON() ([]byte, error) {
 		MLATSources:     a.MLATSources,
 		PositionSource:  a.PositionSource,
 		UpdateRate:      a.UpdateRate,
+		Country:         a.Country,
+		CountryCode:     a.CountryCode,
+		Flag:            a.Flag,
 	})
 }