v0.3.5: Bug修复+性能优化+状态栏文档统计

Bug修复:
- BUG-01: Sidebar norm函数正则修复(单反斜杠匹配), Windows路径规范化恢复
- BUG-02: 3处result.content truthy检查改为content !== undefined, 空文件可打开
- BUG-03: TAB_SWITCHED路径校验顺序修复, activeFilePath不在校验前写入

优化:
- OPT-01: Sidebar handleFileClick添加loading状态, 打开文件有视觉反馈

新功能:
- FEAT-01: 状态栏添加行数/单词数/字符数统计
  - useDocStats hook + computeDocStats 纯函数
  - StatusBar右侧显示 行/词/字符 统计信息
  - 12个单元测试覆盖空值/空白/ASCII/UTF-8/Markdown等场景

验证: TS 0错误, ESLint 0错误(1个预存警告), 90/90测试通过

src/main/ipc-handlers.ts

@@ -197,15 +197,16 @@ export function registerIpcHandlers(
 
   // 标签切换
   ipcMain.handle(IPC_CHANNELS.TAB_SWITCHED, (_event: IpcMainInvokeEvent, filePath: string | null) => {
-    state.activeFilePath = filePath || null
-    if (filePath && !validatePath(filePath)) {
+    const normalizedPath = filePath || null
+    if (normalizedPath && !validatePath(normalizedPath)) {
       fileWatcher.stop()
       return
     }
-    fileWatcher.start(filePath || '')
+    state.activeFilePath = normalizedPath
+    fileWatcher.start(normalizedPath || '')
     const win = getMainWindow()
     if (win && !win.isDestroyed()) {
-      win.setTitle(filePath ? `MarkLite - ${basename(filePath)}` : 'MarkLite')
+      win.setTitle(normalizedPath ? `MarkLite - ${basename(normalizedPath)}` : 'MarkLite')
     }
   })