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/Package.appxmanifest

@@ -37,11 +37,11 @@
       <uap:VisualElements
         DisplayName="AmiReel"
         Description="AmiReel"
-        BackgroundColor="transparent"
+        BackgroundColor="#0E1525"
         Square150x150Logo="Assets\Square150x150Logo.png"
         Square44x44Logo="Assets\Square44x44Logo.png">
         <uap:DefaultTile Wide310x150Logo="Assets\Wide310x150Logo.png" />
-        <uap:SplashScreen Image="Assets\SplashScreen.png" />
+        <uap:SplashScreen Image="Assets\SplashScreen.png" BackgroundColor="#0E1525" />
       </uap:VisualElements>
     </Application>
   </Applications>