fix(export): drop invalid file-name characters from the suggested export name

A workspace at a drive root is named "D:\", which made the suggested file name
invalid. Characters that are invalid in a file name on any supported platform are
now removed, and a name left empty (a workspace at /) falls back to YKanBan.

YKanBan.Tests/Export/MarkdownExporterTests.cs

@@ -181,6 +181,21 @@ public class MarkdownExporterTests
         }
     }
 
+    [TestMethod]
+    [DataRow("D:\\", "D")]
+    [DataRow("a<b>c|d?e*f\"g", "abcdefg")]
+    [DataRow("/", null)]
+    public void FileNameDropsCharactersThatAreInvalidInFileNames(string workspaceName, string? expectedName)
+    {
+        string fileName = MarkdownExporter.BuildFileName(workspaceName, ExportedAt);
+
+        CultureInfo culture = Resources.Culture ?? CultureInfo.CurrentUICulture;
+        Assert.AreEqual(
+            string.Format(culture, Resources.Export_FileNamePattern, expectedName ?? Resources.App_Name, ExportedAt),
+            fileName);
+        Assert.AreEqual(-1, fileName.IndexOfAny(Path.GetInvalidFileNameChars()));
+    }
+
     [TestMethod]
     public void TagTableCellsEscapePipesAndLineBreaks()
     {