Improve aircraft icons and add weight-based color coding

- Replaced simple geometric shapes with detailed, realistic SVG icons for all aircraft types
- Implemented color differentiation based on aircraft weight categories:
  * Light (<7000kg): Sky blue
  * Medium (7000-34000kg): Green
  * Large (34000-136000kg): Orange
  * Heavy (>136000kg): Red
- Created new icons for: commercial airliner, helicopter, cargo aircraft, general aviation, military fighter, ground vehicle
- Updated legend to reflect new weight-based categories
- Modified getAircraftColor() to assign colors based on aircraft category/weight

Closes #17

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

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

internal/modes/decoder.go

@@ -232,6 +232,11 @@ func (d *Decoder) Decode(data []byte) (*Aircraft, error) {
 
 	df := (data[0] >> 3) & 0x1F
 	icao := d.extractICAO(data, df)
+	
+	// Debug: Log DF types to verify we're processing new formats
+	if df == 0 || df == 11 || df == 16 || df == 19 || df == 24 {
+		fmt.Printf("DEBUG: Processing new DF%d message for ICAO %06X\n", df, icao)
+	}
 
 	aircraft := &Aircraft{
 		ICAO24: icao,