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/modes/decoder.go

@@ -130,8 +130,9 @@ type Aircraft struct {
 	Heading      int // Aircraft heading in degrees (magnetic, integer)
 
 	// Aircraft Information
-	Category string // Aircraft category (size, type, performance)
-	Squawk   string // 4-digit transponder squawk code (octal)
+	Category          string // Aircraft category (size, type, performance)
+	Squawk            string // 4-digit transponder squawk code (octal)
+	SquawkDescription string // Human-readable description of transponder code
 
 	// Status and Alerts
 	Emergency string // Emergency/priority status description