Fix MVVMTK0034 warning from CommunityToolkit.Mvvm 8.4.0 upgrade

[Copilot]

CommunityToolkit.Mvvm 8.4.0 enforces stricter rules against directly accessing [ObservableProperty] backing fields, causing a build warning in SettingsViewModel.cs.

Changes

• Added _isLoading flag to prevent OnThemeChanged callback during settings initialization
• Updated LoadSettings() to use generated Theme property instead of _theme field
• Wrapped flag in try-finally for proper cleanup on exceptions

partial void OnThemeChanged(string value)
{
    if (_isLoading) return;
    App.Instance?.ApplyTheme(value);
}

private void LoadSettings()
{
    try
    {
        _isLoading = true;
        Theme = Preferences.Theme; // Uses property, not _theme field
        // ...
    }
    finally
    {
        _isLoading = false;
    }
}

This maintains the original behavior of skipping theme preview during load while complying with the toolkit's code generation patterns.

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[Copilot]

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