fix(workspace): present AppKit folder picker

Sources/SaplingApp/SaplingApp.swift

@@ -66,7 +66,6 @@ private final class SaplingAppModel: ObservableObject {
     @Published var editorViewModel: HybridMarkdownEditorViewModel?
     @Published var configuration: SaplingConfiguration
     @Published var editorErrorMessage: String?
-    @Published var isImportingWorkspace = false
 
     private let configurationStore: any ConfigurationStore
     private let workspaceManager: any WorkspaceManaging
@@ -92,7 +91,25 @@ private final class SaplingAppModel: ObservableObject {
     }
 
     func presentWorkspaceImporter() {
-        isImportingWorkspace = true
+        #if os(macOS)
+        let panel = NSOpenPanel()
+        panel.title = "Open Workspace"
+        panel.message = "Choose a folder to use as your Sapling workspace."
+        panel.prompt = "Open"
+        panel.canChooseDirectories = true
+        panel.canChooseFiles = false
+        panel.allowsMultipleSelection = false
+        panel.canCreateDirectories = false
+        panel.resolvesAliases = true
+
+        guard panel.runModal() == .OK, let url = panel.url else { return }
+
+        Task {
+            await openWorkspace(at: url)
+        }
+        #else
+        editorErrorMessage = "Workspace selection is currently available on macOS."
+        #endif
     }
 
     func openWorkspace(at url: URL) async {
@@ -249,21 +266,6 @@ private struct MainWindow: View {
                 .keyboardShortcut("s", modifiers: .command)
             }
         }
-        .fileImporter(
-            isPresented: $model.isImportingWorkspace,
-            allowedContentTypes: [.folder],
-            allowsMultipleSelection: false
-        ) { result in
-            switch result {
-            case .success(let urls):
-                guard let url = urls.first else { return }
-                Task {
-                    await model.openWorkspace(at: url)
-                }
-            case .failure(let error):
-                model.editorErrorMessage = "Unable to open workspace: \(error.localizedDescription)"
-            }
-        }
         .fileImporter(
             isPresented: $isImportingMarkdown,
             allowedContentTypes: [.saplingMarkdown, .plainText],