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/components/OutlinePanel/OutlinePanel.tsx

@@ -0,0 +1,99 @@
+import React, { memo } from 'react'
+
+// --- Types ---
+
+export interface Heading {
+  level: number
+  text: string
+  /** Position in document (character offset from markdown source) */
+  pos: number
+}
+
+// --- Heading Parser ---
+
+const HEADING_RE = /^(#{1,6})\s+(.+)$/gm
+
+/**
+ * Parse headings from raw markdown content using regex.
+ */
+export function parseHeadings(markdown: string): Heading[] {
+  const headings: Heading[] = []
+  let match: RegExpExecArray | null
+
+  // Reset regex state
+  HEADING_RE.lastIndex = 0
+
+  while ((match = HEADING_RE.exec(markdown)) !== null) {
+    const level = match[1].length
+    const text = match[2].trim()
+    headings.push({ level, text, pos: match.index })
+  }
+
+  return headings
+}
+
+// --- Component ---
+
+interface OutlinePanelProps {
+  headings: Heading[]
+  onNavigate: (pos: number) => void
+  activeHeadingIndex: number | null
+}
+
+interface OutlineItemProps {
+  heading: Heading
+  isActive: boolean
+  onNavigate: (pos: number) => void
+}
+
+const OutlineItem = memo(function OutlineItem({
+  heading,
+  isActive,
+  onNavigate
+}: OutlineItemProps) {
+  return (
+    <button
+      className={`outline-item outline-level-${heading.level}${isActive ? ' active' : ''}`}
+      onClick={() => onNavigate(heading.pos)}
+      title={heading.text}
+      aria-label={`跳转到标题：${heading.text}`}
+      style={{ paddingLeft: `${8 + (heading.level - 1) * 12}px` }}
+    >
+      <span className="outline-level-dot" />
+      <span className="outline-item-text">{heading.text}</span>
+    </button>
+  )
+})
+
+export const OutlinePanel = memo(function OutlinePanel({
+  headings,
+  onNavigate,
+  activeHeadingIndex
+}: OutlinePanelProps) {
+  if (headings.length === 0) {
+    return (
+      <div className="outline-panel" role="region" aria-label="文档大纲">
+        <div className="outline-header">文档大纲</div>
+        <div className="outline-empty">当前文档无标题</div>
+      </div>
+    )
+  }
+
+  return (
+    <div className="outline-panel" role="region" aria-label="文档大纲">
+      <div className="outline-header">文档大纲</div>
+      <div className="outline-list" role="list" aria-label="标题列表">
+        {headings.map((h, i) => (
+          <OutlineItem
+            key={`${h.text}-${h.pos}`}
+            heading={h}
+            isActive={i === activeHeadingIndex}
+            onNavigate={onNavigate}
+          />
+        ))}
+      </div>
+    </div>
+  )
+})
+
+OutlinePanel.displayName = 'OutlinePanel'