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/main/file-system.ts

@@ -1,13 +1,9 @@
 import { readFile, stat, writeFile, rename, readdir, unlink } from 'fs/promises'
 import { join, extname, dirname, basename } from 'path'
 import type { ReadFileResult, SaveFileResult, FileNode } from '../shared/types'
+import { MAX_FILE_SIZE, ALLOWED_EXTENSIONS, SKIP_DIRS } from '../shared/constants'
 
-const MAX_FILE_SIZE = 20 * 1024 * 1024 // 20MB
-const ALLOWED_EXTENSIONS = new Set(['.md', '.markdown', '.txt'])
-const SKIP_DIRS = new Set([
-  'node_modules', '.git', '.svn', '.hg', 'dist', 'out',
-  '.next', '.nuxt', '__pycache__', '.DS_Store'
-])
+const ALLOWED_EXTENSIONS_SET = new Set<string>(ALLOWED_EXTENSIONS)
 
 export async function readFileContent(filePath: string): Promise<ReadFileResult> {
   try {
@@ -67,7 +63,7 @@ export async function buildDirTree(dirPath: string, depth = 0, maxDepth = 10): P
       }
     } else {
       const ext = extname(entry.name).toLowerCase()
-      if (ALLOWED_EXTENSIONS.has(ext)) {
+      if (ALLOWED_EXTENSIONS_SET.has(ext)) {
         children.push({ name: entry.name, path: childPath, type: 'file' })
       }
     }