Thread safety issues with SharedPreferences

[callebtc]

Description:
Multiple SharedPreferences files are accessed from different threads without proper synchronization, creating race conditions.

Affected Files:

• ThemeManager.kt: "app_prefs"
• CashuWalletManager.kt: "CashuWalletPrefs"
• ModernPOSActivity.kt: "NumoPrefs"
• AmountDisplayManager.kt: preference access

Issues:

1. Inconsistent preference file naming
2. No synchronization for concurrent access
3. Potential data corruption in multi-threaded scenarios

Fix Instructions:
Option 1 - Add synchronization:

@Synchronized
private fun getPrefs(): SharedPreferences {
    return context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE)
}

Option 2 - Migrate to DataStore (recommended):

// Replace SharedPreferences with Jetpack DataStore
implementation "androidx.datastore:datastore-preferences:1.0.0"

Recommendation: Migrate to DataStore for better thread safety and async support.