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.cs

@@ -1,8 +1,13 @@
+using Microsoft.UI;
 using Microsoft.UI.Windowing;
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
 using System;
 using System.IO;
+using System.Runtime.InteropServices;
+using Windows.Graphics;
+using Windows.UI;
+using WinRT.Interop;
 
 // To learn more about WinUI, the WinUI project structure,
 // and more about our project templates, see: http://aka.ms/winui-project-info.
@@ -21,6 +26,7 @@ public sealed partial class MainWindow : Window
     public MainWindow()
     {
         InitializeComponent();
+        App.MainWindowInstance = this;
 
         ExtendsContentIntoTitleBar = true;
         SetTitleBar(AppTitleBar);
@@ -29,26 +35,77 @@ public sealed partial class MainWindow : Window
         if (!string.IsNullOrWhiteSpace(executablePath) && File.Exists(executablePath))
             AppWindow.SetIcon(executablePath);
 
+        SetDefaultSizeAndPosition();
+
         // Navigate the root frame to the main page on startup.
         RootFrame.Navigate(typeof(MainPage));
 
         AppWindow.Closing += AppWindow_Closing;
     }
 
+    private void SetDefaultSizeAndPosition()
+    {
+        const int DefaultWidth = 1280;
+        const int DefaultHeight = 860;
+        const int MinWidth = 1100;
+        const int MinHeight = 780;
+
+        IntPtr hwnd = WindowNative.GetWindowHandle(this);
+        double scale = GetDpiForWindow(hwnd) / 96.0;
+        int width = (int)(DefaultWidth * scale);
+        int height = (int)(DefaultHeight * scale);
+
+        AppWindow.Resize(new SizeInt32(width, height));
+
+        if (AppWindow.Presenter is OverlappedPresenter presenter)
+        {
+            presenter.PreferredMinimumWidth = (int)(MinWidth * scale);
+            presenter.PreferredMinimumHeight = (int)(MinHeight * scale);
+        }
+
+        DisplayArea? displayArea = DisplayArea.GetFromWindowId(Win32Interop.GetWindowIdFromWindow(hwnd), DisplayAreaFallback.Primary);
+        if (displayArea is not null)
+        {
+            int centerX = displayArea.WorkArea.X + (displayArea.WorkArea.Width - width) / 2;
+            int centerY = displayArea.WorkArea.Y + (displayArea.WorkArea.Height - height) / 2;
+            AppWindow.Move(new PointInt32(centerX, centerY));
+        }
+    }
+
+    [DllImport("user32.dll")]
+    private static extern int GetDpiForWindow(IntPtr hwnd);
+
+    public void ApplyTheme(bool isDark)
+    {
+        if (Content is FrameworkElement root)
+            root.RequestedTheme = isDark ? ElementTheme.Dark : ElementTheme.Light;
+
+        AppWindowTitleBar titleBar = AppWindow.TitleBar;
+        Color foreground = isDark ? Color.FromArgb(255, 0xF5, 0xF7, 0xFB) : Color.FromArgb(255, 0x12, 0x20, 0x33);
+        Color hoverBackground = isDark ? Color.FromArgb(255, 0x2E, 0x44, 0x6C) : Color.FromArgb(255, 0xD7, 0xE4, 0xF4);
+
+        titleBar.ButtonBackgroundColor = Colors.Transparent;
+        titleBar.ButtonInactiveBackgroundColor = Colors.Transparent;
+        titleBar.ButtonForegroundColor = foreground;
+        titleBar.ButtonInactiveForegroundColor = foreground;
+        titleBar.ButtonHoverBackgroundColor = hoverBackground;
+        titleBar.ButtonHoverForegroundColor = foreground;
+        titleBar.ButtonPressedBackgroundColor = hoverBackground;
+        titleBar.ButtonPressedForegroundColor = foreground;
+    }
+
     private async void AppWindow_Closing(AppWindow sender, AppWindowClosingEventArgs args)
     {
         if (_exitConfirmed) return;
         args.Cancel = true;
 
-        ContentDialog dialog = new()
-        {
-            Title = "Exit AmiReel?",
-            Content = "Are you sure you want to close the application?",
-            PrimaryButtonText = "Exit",
-            CloseButtonText = "Cancel",
-            DefaultButton = ContentDialogButton.Close,
-            XamlRoot = RootFrame.XamlRoot
-        };
+        FrameworkElement root = (FrameworkElement)Content;
+        ContentDialog dialog = DialogHelper.CreateStyled(RootFrame.XamlRoot, root.RequestedTheme);
+        dialog.Title = "Exit AmiReel?";
+        dialog.Content = "Are you sure you want to close the application?";
+        dialog.PrimaryButtonText = "Exit";
+        dialog.CloseButtonText = "Cancel";
+        dialog.DefaultButton = ContentDialogButton.Primary;
 
         ContentDialogResult result = await dialog.ShowAsync();
         if (result != ContentDialogResult.Primary) return;