✨ Feature Request: Global Default Storage Configuration

[moazelsawaf]

Hi @SPiercer 👋🏻

Thank you for creating this excellent package! I think this enhancement would make it even more convenient to use 🙏🏻

🤔 Problem

Currently, every Saveable value requires passing the storage parameter:

final counter = Saveable<int>(
  key: 'counter',
  storage: myStorage,  // ← Repetitive 😓
  defaultValue: 0,
);

final username = Saveable<String>(
  key: 'username',
  storage: myStorage,  // ← Repetitive 😓
  defaultValue: '',
);

final isLoggedIn = Saveable<bool>(
  key: 'is_logged_in',
  storage: myStorage,  // ← Repetitive 😓
  defaultValue: false,
);

This becomes verbose and repetitive in apps with many saveable values.

💡 Proposed Solution

Configure a default storage once at app startup, while keeping it optional for value-specific overrides:

// Configure once at startup ⚙️
void main() async {
  final prefs = await SharedPreferences.getInstance();
  final storage = SharedPreferencesStorage(prefs);
  
  Saveable.setDefaultStorage(storage);  // ← Global configuration ✨
  
  runApp(MyApp());
}

// Use without storage parameter 🎉
final counter = Saveable<int>(
  key: 'counter',
  defaultValue: 0,
  // storage automatically uses default
);

final username = Saveable<String>(
  key: 'username',
  defaultValue: '',
);

// Override for specific values when needed 🔐
final secureToken = Saveable<String>(
  key: 'auth_token',
  storage: secureStorage,  // ← Optional override for sensitive data
);

🎯 Benefits

• Less boilerplate 📉: No need to pass storage for every value
• Cleaner code ✨: Focus on the value configuration, not storage plumbing
• Flexible 🔄: Can still override storage for specific values (e.g., secure storage for tokens)
• Easier refactoring 🛠️: Change default storage in one place