fix(export): write through the storage provider when the target has no local path

The save picker used to return only TryGetLocalPath(), so a target without a
local path looked like a cancel and nothing was written. The dialog service now
hands back an opener for the chosen file. The picker call is inside the error
handler, and the workspace is read before the target is opened, so a failed read
leaves an existing file untouched.

YKanBan.Tests/ViewModels/TagsAndSearchViewModelTests.cs

@@ -414,5 +414,22 @@ public class TagsAndSearchViewModelTests
         Assert.IsTrue(message.HasDetails);
     }
 
+    [TestMethod]
+    public async Task ExportReadFailureIsReportedAndLeavesTheTargetUntouched()
+    {
+        using var fixture = new Fixture();
+        WorkspaceViewModel workspace = fixture.CreateWorkspace();
+        string path = Path.Combine(fixture.Directory.FullPath, "out.md");
+        File.WriteAllText(path, "previous");
+        fixture.Dialogs.SaveFilePath = path;
+
+        fixture.Repository.Dispose();
+        await workspace.ExportCommand.ExecuteAsync(null);
+
+        var message = (MessageDialogViewModel)fixture.Dialogs.Shown.Single();
+        Assert.AreEqual(Resources.Export_Failed_Title, message.Title);
+        Assert.AreEqual("previous", File.ReadAllText(path));
+    }
+
     #endregion
 }