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/DialogHelper.cs

@@ -0,0 +1,33 @@
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Media;
+
+namespace AmiReel_WinUI;
+
+/// <summary>
+/// Builds ContentDialogs styled to match the app's own palette and button shapes,
+/// since ContentDialog's default Fluent look (accent-pill primary button, plain
+/// square close button, ungrouped theme resources) does not follow our custom brushes.
+/// </summary>
+internal static class DialogHelper
+{
+    public static ContentDialog CreateStyled(XamlRoot xamlRoot, ElementTheme theme)
+    {
+        string themeKey = theme == ElementTheme.Dark ? "Dark" : "Light";
+        ResourceDictionary themeDictionary = (ResourceDictionary)Application.Current.Resources.ThemeDictionaries[themeKey];
+
+        return new ContentDialog
+        {
+            XamlRoot = xamlRoot,
+            RequestedTheme = theme,
+            Background = (Brush)themeDictionary["PanelBrush"],
+            Foreground = (Brush)themeDictionary["TextBrush"],
+            BorderBrush = (Brush)themeDictionary["HeroBorderBrush"],
+            BorderThickness = new Thickness(1.5),
+            CornerRadius = new CornerRadius(18),
+            PrimaryButtonStyle = (Style)Application.Current.Resources["DialogPrimaryButtonStyle"],
+            SecondaryButtonStyle = (Style)Application.Current.Resources["DialogCloseButtonStyle"],
+            CloseButtonStyle = (Style)Application.Current.Resources["DialogCloseButtonStyle"]
+        };
+    }
+}