AsyncNotifier wrapping flutter_secure_storage's "theme_mode" key.
Returns AppThemeMode.system as the default. set(mode) writes the
value + updates state.
Used by MinstrelApp (next commit) to drive MaterialApp.themeMode and
by the Settings appearance section to persist the operator's pick.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
