Add Shorts wizard and v4 video-style pipeline

Turn the Shorts tab into a step-by-step wizard and port
amigadb-short-fx-v4.sh: Video background style, auto/manual crop,
source fade-in, and blur/opacity/margin settings.

App.xaml

@@ -181,6 +181,12 @@
                 <Setter Property="CornerRadius" Value="10" />
             </Style>
 
+            <Style x:Key="TabButtonStyle" TargetType="Button" BasedOn="{StaticResource DefaultButtonStyle}">
+                <Setter Property="Padding" Value="16,9" />
+                <Setter Property="FontSize" Value="13" />
+                <Setter Property="CornerRadius" Value="10" />
+            </Style>
+
             <Style x:Key="PrimaryButtonStyle" TargetType="Button">
                 <Setter Property="Background" Value="{ThemeResource AccentBrush}" />
                 <Setter Property="Foreground" Value="{ThemeResource PrimaryTextBrush}" />