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>

Models/SupportedVideoFormats.cs

@@ -0,0 +1,23 @@
+using System.IO;
+using System.Linq;
+
+namespace AmigaDB.VideoRenderer.Models;
+
+/// <summary>
+/// Video file extensions accepted as render sources. FFmpeg can demux far more than this,
+/// but this list covers what a screen/video capture workflow is realistically going to produce.
+/// </summary>
+public static class SupportedVideoFormats
+{
+    public static readonly IReadOnlyList<string> Extensions =
+    [
+        ".avi", ".mp4", ".m4v", ".mov", ".mkv", ".webm",
+        ".wmv", ".flv", ".mpg", ".mpeg", ".ts", ".3gp"
+    ];
+
+    public static bool IsSupported(string path) =>
+        Extensions.Contains(Path.GetExtension(path), StringComparer.OrdinalIgnoreCase);
+
+    /// <summary>Picker filter string, e.g. "*.avi;*.mp4;*.m4v;...".</summary>
+    public static string PickerFilter => string.Join(';', Extensions.Select(e => "*" + e));
+}