Migrate deprecated `Key? key` constructor pattern to `super.key`

## Description

All screens and widgets use the old constructor pattern:
```dart
const MyWidget({Key? key}) : super(key: key);
```

Dart 3+ supports the cleaner `super.key` syntax:
```dart
const MyWidget({super.key});
```

## Affected Files
All files in `lib/screens/` and `lib/widgets/`.

## Priority
Low — cosmetic, no functional impact. Good for a refactor pass.