Standardize SharedPreferences naming across app

[callebtc]

Description:
The app uses inconsistent naming for SharedPreferences files, creating maintenance confusion and potential refactoring risks.

Current Inconsistent Names:

• "NumoPrefs" (ModernPOSActivity.kt)
• "app_prefs" (ThemeManager.kt)
• "CashuWalletPrefs" (CashuWalletManager.kt)
• Various other preference files

Issues:

1. Maintenance confusion
2. Potential data loss during refactoring
3. No clear naming convention

Fix Instructions:

1. Choose a standard naming pattern (e.g., "numo_" prefix)
2. Update all preference file names:

// Standardize to:
private const val PREFS_NAME = "numo_main_prefs"
private const val WALLET_PREFS_NAME = "numo_wallet_prefs"  
private const val THEME_PREFS_NAME = "numo_theme_prefs"

3. Add migration code to preserve existing user settings:

// Migration example:
private fun migratePreferences() {
    val oldPrefs = getSharedPreferences("app_prefs", MODE_PRIVATE)
    val newPrefs = getSharedPreferences("numo_theme_prefs", MODE_PRIVATE)
    
    if (oldPrefs.all.isNotEmpty() && newPrefs.all.isEmpty()) {
        // Migrate old preferences to new file
    }
}

Priority: Medium - affects maintainability