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()
     {

YKanBan/Export/MarkdownExporter.cs

@@ -67,7 +67,10 @@ public static class MarkdownExporter
 
     /// <summary>
     /// Builds the default export file name in the UI language, for example
-    /// "MyRepo-export-20260930-142537.md".
+    /// "MyRepo-export-20260930-142537.md". Characters that are not allowed in a
+    /// file name on any supported platform are dropped from the workspace name
+    /// (a workspace at a drive root is named "D:\"); a name left empty falls
+    /// back to the application name.
     /// </summary>
     /// <param name="workspaceName">Workspace display name.</param>
     /// <param name="exportedAt">Export moment.</param>
@@ -75,8 +78,22 @@ public static class MarkdownExporter
     public static string BuildFileName(string workspaceName, DateTimeOffset exportedAt)
     {
         CultureInfo culture = Resources.Culture ?? CultureInfo.CurrentUICulture;
-        return string.Format(culture, Resources.Export_FileNamePattern, workspaceName, exportedAt);
+        string safeName = new(workspaceName.Where(character => !IsInvalidFileNameCharacter(character)).ToArray());
+        if (safeName.Length == 0)
+        {
+            safeName = Resources.App_Name;
         }
+        return string.Format(culture, Resources.Export_FileNamePattern, safeName, exportedAt);
+    }
+
+    /// <summary>
+    /// Tells whether a character is invalid in a file name on Windows, Linux or
+    /// macOS; the Windows set is the widest, so the result is the same on every platform.
+    /// </summary>
+    /// <param name="character">The character to test.</param>
+    /// <returns><see langword="true"/> when the character must not appear in a file name.</returns>
+    private static bool IsInvalidFileNameCharacter(char character) =>
+        character < ' ' || character is '<' or '>' or ':' or '"' or '/' or '\\' or '|' or '?' or '*';
 
     /// <summary>
     /// Appends one card block: heading, body, tags line and timestamps line.

docs/PLAN.md

@@ -16,7 +16,7 @@
 - 网络共享明确不支持；命令行参数边界；macOS 无参启动行为写入文档（§2.3、§4.1、附录）
 - 补充：导出转义规则、卡片列表虚拟化、模态窗口 FlowDirection、`Resources` 类名使用规则、CI osx-x64 运行器（§5、§7、§8）
 - 依赖：Avalonia 11.3.6 → 11.3.22，Semi.Avalonia 11.2.1.10 → 11.3.22（§7）
-- 实现回补：搜索括号嵌套上限 64 层、单个表达式至多 256 个原子，空短语、短语紧贴文本、`key:` 后空白均为非法（§6.1、§6.2）；导出表格单元格转义 `\`（§5.8）；初始化中途锁冲突进锁冲突页、数据库文件缺失不静默新建（§4.4）；app.json 读不出 / 备份写不出 / 退出写盘失败均不阻断启动与退出（§4.3）
+- 实现回补：搜索括号嵌套上限 64 层、单个表达式至多 256 个原子，空短语、短语紧贴文本、`key:` 后空白均为非法（§6.1、§6.2）；导出表格单元格转义 `\`、默认文件名剔除非法字符（§5.8）；初始化中途锁冲突进锁冲突页、数据库文件缺失不静默新建（§4.4）；app.json 读不出 / 备份写不出 / 退出写盘失败均不阻断启动与退出（§4.3）
 - 里程碑：标注 M0–M4 已完成，新增 **M4R（v1.9 回补）**（§9）
 
 ## 1. 项目概述
@@ -223,7 +223,7 @@ card_tags: (card_id, tag_id) 复合主键，双向 ON DELETE CASCADE
 
 ### 5.8 导出
 
-- 点击后弹系统保存文件对话框，默认文件名按当前语言生成（模式为资源键，**精确到秒**）：英文 `MyRepo-export-20260930-142537.md`；中文 `MyRepo-导出-20260930-142537.md`
+- 点击后弹系统保存文件对话框，默认文件名按当前语言生成（模式为资源键，**精确到秒**）：英文 `MyRepo-export-20260930-142537.md`；中文 `MyRepo-导出-20260930-142537.md`；工作区名中任一平台的文件名非法字符（`<>:"/\|?*` 与控制字符）剔除，剔除后为空（如工作区位于 `/`）则用 `YKanBan`
 - 人类可读格式，无回导需求；模板文本直接读取 ResX 资源（见 §8），导出语言跟随当前界面语言
 - 导出范围为工作区全量数据，不受当前搜索筛选与卡片排序选项影响：列按 id 升序，列内卡片按 id 升序
 - 全文时间戳（导出时间与卡片创建/修改时间）格式随语言：英文 `yyyy-MM-dd HH:mm:ss`；中文 `yyyy年M月d日 HH:mm`