Update aircraft legend to show real ADS-B categories and implement pure Go ICAO country lookup

- Replace imaginary aircraft types (Commercial/Cargo/GA) with actual ADS-B emitter categories
- Add proper weight-based classifications: Light <7000kg, Medium 7000-34000kg, etc.
- Replace SQLite-based ICAO lookup with pure Go implementation using slice of allocations
- Remove SQLite dependency completely for simpler architecture
- Add comprehensive ICAO address allocations based on ICAO Document 8585
- Implement efficient linear search through sorted allocations by start address

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

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

internal/merger/merger.go

@@ -317,7 +317,8 @@ func (m *Merger) UpdateAircraft(sourceID string, aircraft *modes.Aircraft, signa
 		}
 		
 		// Lookup country information for new aircraft
-		if countryInfo, err := m.icaoDB.LookupCountry(fmt.Sprintf("%06X", aircraft.ICAO24)); err == nil {
+		icaoHex := fmt.Sprintf("%06X", aircraft.ICAO24)
+		if countryInfo, err := m.icaoDB.LookupCountry(icaoHex); err == nil {
 			state.Country = countryInfo.Country
 			state.CountryCode = countryInfo.CountryCode
 			state.Flag = countryInfo.Flag