Fix DataGrid row/cell colors not adapting to theme

[Copilot]

WPF's DataGridRow and DataGridCell have their own system default templates and do not inherit Background/Foreground from the parent DataGrid, so rows and cells stayed white/black regardless of theme.

Changes

• Added RowStyle to ThemedDataGrid: binds Background/Foreground to PanelBackgroundBrush/WindowForegroundBrush; selected and hovered rows use AccentBrushSubtle
• Added CellStyle to ThemedDataGrid: transparent background (row controls the fill), Foreground bound to WindowForegroundBrush, selected cells render white text with transparent border to avoid the default blue selection highlight clashing with row-level highlight

<Setter Property="RowStyle">
    <Setter.Value>
        <Style TargetType="DataGridRow">
            <Setter Property="Background" Value="{DynamicResource PanelBackgroundBrush}" />
            <Setter Property="Foreground" Value="{DynamicResource WindowForegroundBrush}" />
            ...
        </Style>
    </Setter.Value>
</Setter>
<Setter Property="CellStyle">
    <Setter.Value>
        <Style TargetType="DataGridCell">
            <Setter Property="Background" Value="Transparent" />
            <Setter Property="Foreground" Value="{DynamicResource WindowForegroundBrush}" />
            <Setter Property="BorderBrush" Value="Transparent" />
            ...
        </Style>
    </Setter.Value>
</Setter>