fix: open folder pickers at current paths

src/MBN_STOCK_WEBVIEW.LegacyParityApp/MainWindow.BackgroundPicker.cs

@@ -17,7 +17,7 @@ public sealed partial class MainWindow
             try
             {
                 completion.TrySetResult(
-                    LegacyBackgroundFileDialog.Show(
+                    LegacyFileDialog.ShowBackgroundFile(
                         WindowNative.GetWindowHandle(this),
                         initialDirectory));
             }
@@ -40,14 +40,54 @@ public sealed partial class MainWindow
         return completion.Task;
     }
 
-    private static class LegacyBackgroundFileDialog
+    private Task<string?> PickOperatorFolderAsync(
+        string? initialDirectory,
+        string title)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(title);
+        var canonicalInitialDirectory = initialDirectory is null
+            ? null
+            : Path.GetFullPath(initialDirectory);
+        var completion = new TaskCompletionSource<string?>(
+            TaskCreationOptions.RunContinuationsAsynchronously);
+
+        void ShowPicker()
+        {
+            try
+            {
+                completion.TrySetResult(
+                    LegacyFileDialog.ShowFolder(
+                        WindowNative.GetWindowHandle(this),
+                        canonicalInitialDirectory,
+                        title));
+            }
+            catch (Exception exception)
+            {
+                completion.TrySetException(exception);
+            }
+        }
+
+        if (DispatcherQueue.HasThreadAccess)
+        {
+            ShowPicker();
+        }
+        else if (!DispatcherQueue.TryEnqueue(ShowPicker))
+        {
+            completion.TrySetException(
+                new InvalidOperationException("The folder picker could not reach the UI thread."));
+        }
+
+        return completion.Task;
+    }
+
+    private static class LegacyFileDialog
     {
         private const int CancelledHResult = unchecked((int)0x800704C7);
         private const uint FileSystemPathDisplayName = 0x80058000;
         private static readonly Guid FileOpenDialogClassId =
             new("DC1C5A9C-E88A-4DDE-A5A1-60F82A20AEF7");
 
-        public static string? Show(nint owner, string initialDirectory)
+        public static string? ShowBackgroundFile(nint owner, string initialDirectory)
         {
             if (owner == nint.Zero || !Directory.Exists(initialDirectory))
             {
@@ -127,6 +167,89 @@ public sealed partial class MainWindow
             }
         }
 
+        public static string? ShowFolder(
+            nint owner,
+            string? initialDirectory,
+            string title)
+        {
+            if (owner == nint.Zero)
+            {
+                throw new InvalidOperationException(
+                    "The folder picker requires an owner window.");
+            }
+
+            if (initialDirectory is not null &&
+                !Directory.Exists(initialDirectory))
+            {
+                throw new DirectoryNotFoundException(
+                    "The initial folder-picker directory is unavailable.");
+            }
+
+            IFileDialog? dialog = null;
+            IShellItem? initialFolder = null;
+            IShellItem? result = null;
+            try
+            {
+                var dialogType = Type.GetTypeFromCLSID(
+                    FileOpenDialogClassId,
+                    throwOnError: true) ??
+                    throw new InvalidOperationException(
+                        "The Windows folder dialog is unavailable.");
+                dialog = (IFileDialog)(Activator.CreateInstance(dialogType) ??
+                    throw new InvalidOperationException(
+                        "The Windows folder dialog could not be created."));
+                dialog.GetOptions(out var existingOptions);
+                dialog.SetOptions(
+                    existingOptions |
+                    FileOpenOptions.NoChangeDirectory |
+                    FileOpenOptions.PickFolders |
+                    FileOpenOptions.ForceFileSystem |
+                    FileOpenOptions.PathMustExist |
+                    FileOpenOptions.NoDereferenceLinks |
+                    FileOpenOptions.DoNotAddToRecent);
+                dialog.SetTitle(title);
+                dialog.SetOkButtonLabel("폴더 선택");
+
+                if (initialDirectory is not null)
+                {
+                    var shellItemId = typeof(IShellItem).GUID;
+                    var createResult = SHCreateItemFromParsingName(
+                        initialDirectory,
+                        nint.Zero,
+                        ref shellItemId,
+                        out initialFolder);
+                    Marshal.ThrowExceptionForHR(createResult);
+                    dialog.SetFolder(initialFolder);
+                    dialog.SetDefaultFolder(initialFolder);
+                }
+
+                var showResult = dialog.Show(owner);
+                if (showResult == CancelledHResult)
+                {
+                    return null;
+                }
+
+                Marshal.ThrowExceptionForHR(showResult);
+                dialog.GetResult(out result);
+                result.GetDisplayName(FileSystemPathDisplayName, out var pathPointer);
+                try
+                {
+                    return Marshal.PtrToStringUni(pathPointer) ??
+                        throw new IOException("The selected folder path is unavailable.");
+                }
+                finally
+                {
+                    Marshal.FreeCoTaskMem(pathPointer);
+                }
+            }
+            finally
+            {
+                ReleaseComObject(result);
+                ReleaseComObject(initialFolder);
+                ReleaseComObject(dialog);
+            }
+        }
+
         private static void ReleaseComObject(object? value)
         {
             if (value is not null && Marshal.IsComObject(value))
@@ -139,6 +262,7 @@ public sealed partial class MainWindow
         private enum FileOpenOptions : uint
         {
             NoChangeDirectory = 0x00000008,
+            PickFolders = 0x00000020,
             ForceFileSystem = 0x00000040,
             PathMustExist = 0x00000800,
             FileMustExist = 0x00001000,