Bring WinUI app to feature/visual parity with WPF, fix drag-and-drop and branding assets

- Restyle Fluent controls (buttons, fields, dialogs, ListView) to match the
  app's navy dark/light palette and consistent corner radii/sizing
- Fix ContentDialog (Settings, exit-confirm, error) styling by binding
  PrimaryButtonStyle/CloseButtonStyle explicitly via new DialogHelper,
  since ContentDialog ignores implicit Button styles and forces accent
  color onto whichever button is DefaultButton
- Remove MicaBackdrop and theme the title bar directly so it no longer
  shows a gray tint mismatched with the app's navy background
- Replace placeholder Assets/*.png (unused VS template art) with the real
  AmiReel logo at all required tile/splash/store sizes
- Set explicit default window size (1280x860, matching the old WPF app)
  with DPI-aware centering and a minimum size, instead of sizing to content
- Fix drag-and-drop silently failing: InputList_Drop needs a
  DragOperationDeferral before the first await or the DataView is torn
  down before GetStorageItemsAsync completes
- Add Preview player path setting (was present in WPF, missing in WinUI)
  and error handling for preview playback
- Accept all ffmpeg-readable video formats (mp4, mov, mkv, webm, ...) for
  file picker and drag-and-drop, not just .avi
- Add exit-confirmation dialog to WinUI, mirroring the WPF app

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

AmiReel.WinUI/MainWindow.xaml

@@ -8,24 +8,22 @@
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
     Title="AmiReel"
     mc:Ignorable="d">
-    <Window.SystemBackdrop>
-        <MicaBackdrop />
-    </Window.SystemBackdrop>
-
-    <Grid>
+    <Grid Background="{ThemeResource PageBrush}">
         <Grid.RowDefinitions>
             <RowDefinition Height="Auto" />
             <RowDefinition Height="*" />
         </Grid.RowDefinitions>
 
-        <Grid x:Name="AppTitleBar" Height="44" Background="Transparent">
+        <Grid x:Name="AppTitleBar" Grid.Row="0" Height="44" Background="{ThemeResource PanelBrush}">
             <StackPanel Orientation="Horizontal" Spacing="10" VerticalAlignment="Center" Margin="14,0,0,0">
                 <Image Source="ms-appx:///Assets/Square44x44Logo.scale-200.png"
                        Width="20"
                        Height="20"
                        Stretch="Uniform" />
                 <TextBlock Text="AmiReel"
-                           FontSize="16"
+                           FontFamily="Bahnschrift"
+                           FontSize="14"
+                           FontWeight="SemiBold"
                            VerticalAlignment="Center" />
             </StackPanel>
         </Grid>