Configurator crashes and Icon Wizard icons fail to render when using a kit desktop export

## Description

When using Font Awesome's **Icon Wizard** (a Pro feature that lets you combine a core icon with a modifier) and adding those icons to a kit, the **desktop kit export** includes them in `metadata/icons.json` with `"styles": ["custom"]`. There are two bugs that prevent these icons from working:

1. The configurator crashes before generating any files
2. Even after working around the crash, the generated `IconDataCustom` class doesn't exist in the package, causing compile errors
3. The kit's custom OTF font is not registered in `pubspec.yaml`, so icons would not render even if the above were fixed

All three issues must be resolved for Icon Wizard icons to work end-to-end.

---

## Bug 1: Configurator crashes with null cast

### Steps to Reproduce
1. Create icons using the [Font Awesome Icon Wizard](https://docs.fontawesome.com/web/add-icons/icon-wizard/) and add them to a kit
2. Export the kit using the Font Awesome desktop app
3. Copy `metadata/icons.json` to `lib/fonts/icons.json`
4. Run `./configurator.sh`

### Error
```
Custom icons.json found, generating files
Unhandled exception:
type 'Null' is not a subtype of type 'List' in type cast
#0 readAndPickMetadata (file:///.../util/lib/main.dart:617:35)
#1 main (file:///.../util/lib/main.dart:138:25)
```

### Root Cause
`util/lib/main.dart` line 617 casts `icon['changes']` directly to `List`, but Icon Wizard icons have no Font Awesome release history so the field is absent:

```json
"solid-bell-circle-plus": {
"label": "solid-bell-circle-plus",
"styles": ["custom"],
"svg": { ... },
"unicode": "e001",
"voted": false
}
```

### Fix
```dart
// util/lib/main.dart:617
for (var v in (icon['changes'] as List? ?? [])) {
versions.add(v);
}
```

### Workaround
Manually add `"changes": []` to each Icon Wizard icon entry in `icons.json` before running the configurator.

---

## Bug 2: `IconDataCustom` class is missing

After working around Bug 1, the configurator correctly generates constants using `IconDataCustom(0xeXXX)` for custom-style icons, but this class does not exist in `lib/src/icon_data.dart`, causing 12 compile errors:

```
error • The method 'IconDataCustom' isn't defined for the type 'FontAwesomeIcons'
error • Const variables must be initialized with a constant value
```

### Fix
Add `IconDataCustom` to `lib/src/icon_data.dart` pointing to the kit font family:

```dart
class IconDataCustom extends IconData {
const IconDataCustom(super.codePoint)
: super(
fontFamily: 'FontAwesomeKit',
fontPackage: 'font_awesome_flutter',
);
}
```

---

## Bug 3: Kit custom font not registered in `pubspec.yaml`

The desktop export includes a kit-specific OTF file (e.g. `Font Awesome Kit {kitId}-Regular-400.otf`) which contains the Icon Wizard icon glyphs. This file must be copied to `lib/fonts/` and registered in `pubspec.yaml`, otherwise icons render as blank boxes.

### Fix
Copy the kit OTF to `lib/fonts/` and add to `pubspec.yaml`:

```yaml
flutter:
fonts:
- family: FontAwesomeKit
fonts:
- asset: lib/fonts/Font-Awesome-Kit-{kitId}-Regular-400.otf
weight: 400
```

The `FontAwesomeKit` family name must match what is used in `IconDataCustom` above.

---

## Verification

After all three fixes, Icon Wizard icons are accessible both via direct reference and the name mapping:

```dart
// Direct
FaIcon(FontAwesomeIcons.solidBellCirclePlus)

// By name (requires --dynamic flag during configurator run)
FaIcon(faIconNameMapping['custom solid-bell-circle-plus']!)
```

Both approaches confirmed working with the manual fixes applied.

## Notes
- The README makes no mention of Icon Wizard icons or kit custom icons — documentation for this workflow would be valuable for Pro users
- Icon Wizard icons use Font Awesome's `fak` prefix and the naming format `[style]-[core-icon]-[modifier]`
- The name mapping key format for custom icons is `'custom {icon-name}'`

## Environment
- font_awesome_flutter: 10.12.0
- Font Awesome Pro 6.7.2 desktop kit export
- macOS