v0.3.4: Bug修复+性能优化+文档大纲功能

🔴 Bug修复 (6):
- Sidebar反斜杠正则修复 (Windows路径规范化)
- FileWatcher文件删除后自动恢复监听
- SearchReplace replaceAll防过期匹配位置
- openFolderDialog null结果守卫
- 保存异常时空路径保护
- Markdown绝对路径图片拼接修复

🟡 性能优化:
- Editor切换标签时内容比较，相同跳过replaceAll

🔵 代码质量:
- 创建共享常量模块 src/shared/constants.ts，消除双副本

🎯 新增功能:
- 文档大纲 (Table of Contents) — 侧边栏标题导航

🔧 换行符统一: CRLF → LF

src/renderer/hooks/useFolderOperations.ts

@@ -16,6 +16,7 @@ export function useFolderOperations() {
   const handleOpenFolder = useCallback(async () => {
     if (!window.electronAPI) return
     const result = await window.electronAPI.openFolderDialog()
+    if (!result) return
     if (!result.canceled && result.filePaths[0]) {
       const dirPath = result.filePaths[0]
       setRootPath(dirPath)