feat: Add support for double default values

[ikku47]

🐛 Problem

When using the "set default values" option, double fields were not getting default values assigned, causing inconsistent behavior compared to other primitive types (int, bool, String, List).

This could lead to:

• Runtime issues in Dart when double fields are not nullable
• Inconsistent code generation
• Missing default values for double properties

✅ Solution

Added complete support for double default values across the entire codebase:

Changes Made:

1. Settings.kt:

   • Added doubleDefaultValue: String = "0.0" field
   • Added doubleFieldDefaultValue(): String? method

2. ClassDefinition.kt:

   • Added double case in field generation logic
   • Bonus fix: Fixed existing bugs where bool and int checks incorrectly used stringFieldDefaultValue()

3. JsonInputDialog.kt:

   • Added UI input field for double default values
   • Positioned logically between int and bool fields

🧪 Testing

• All files compile without errors
• UI properly displays double default value input field
• Generated code includes double default values when enabled
• Existing functionality for other types remains unchanged

📸 Before vs After

Before: double fields had no default values
After: double fields get configurable default values (default: "0.0")

🔗 Related Issues

Fixes the missing double default value support identified during codebase analysis.