UserDefaults Migrator

Migrate UserDefaults to SwiftData:

1. Find usage: grep -r "UserDefaults.standard"

2. Categorize:

   • Keep: Tests, debug flags, widgets
   • Migrate: User preferences, stats, settings

3. Create entity if needed

4. Update code to use PreferencesStore

5. Write migration logic

6. Archive old keys

Use when: UserDefaults cleanup, preference migration, SwiftData entities