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>

Services/RenderPipeline.cs

@@ -360,8 +360,8 @@ public sealed class RenderPipeline
 
     private static void Validate(RenderSettings s)
     {
-        if (s.InputFiles.Count == 0) throw new ArgumentException("Select at least one AVI input file.");
-        if (s.InputFiles.Any(f => !File.Exists(f))) throw new FileNotFoundException("One or more AVI files no longer exist.");
+        if (s.InputFiles.Count == 0) throw new ArgumentException("Select at least one video input file.");
+        if (s.InputFiles.Any(f => !File.Exists(f))) throw new FileNotFoundException("One or more input files no longer exist.");
         if (!string.IsNullOrWhiteSpace(s.EndCardPath) && !File.Exists(s.EndCardPath))
             throw new FileNotFoundException("End-card image was not found.");
         if (string.IsNullOrWhiteSpace(s.OutputDirectory)) throw new ArgumentException("Select an output directory.");