feat: 替换 textarea 为 CodeMirror 6 编辑器

新增功能：
- CodeMirror 6 专业编辑器替代原生 textarea
- 语法高亮（Markdown 专用）
- 行号（内置，精确同步）
- 代码折叠（foldGutter）
- 括号匹配
- 多光标选区（rectangularSelection）
- 内置搜索高亮（highlightSelectionMatches）
- undo/redo 原生支持（history）
- Tab 缩进（indentWithTab）
- 暗色主题（oneDark）

新增文件：
- useCodeMirror.ts: CodeMirror 初始化 Hook
- EditorToolbar.tsx: 格式化工具栏（B/I/S/标题/列表/引用/代码/链接/图片）
- Editor.tsx: 重写为 CodeMirror 组件

格式化工具栏按钮：
- 粗体/斜体/删除线/行内代码
- H1/H2/H3 标题
- 无序列表/有序列表/引用/代码块
- 链接/图片

快捷键：
- Ctrl+B 粗体
- Ctrl+I 斜体

src/renderer/components/Editor/EditorToolbar.tsx

@@ -0,0 +1,107 @@
+import React, { useCallback } from 'react'
+import type { EditorView } from '@codemirror/view'
+
+interface EditorToolbarProps {
+  viewRef: React.MutableRefObject<EditorView | null>
+}
+
+export function EditorToolbar({ viewRef }: EditorToolbarProps) {
+  const insertFormatting = useCallback((before: string, after: string, placeholder: string) => {
+    const view = viewRef.current
+    if (!view) return
+
+    const { from, to } = view.state.selection.main
+    const selected = view.state.sliceDoc(from, to)
+    const text = selected || placeholder
+    const insert = before + text + after
+
+    view.dispatch({
+      changes: { from, to, insert },
+      selection: { anchor: from + before.length, head: from + before.length + text.length }
+    })
+    view.focus()
+  }, [viewRef])
+
+  const insertLinePrefix = useCallback((prefix: string) => {
+    const view = viewRef.current
+    if (!view) return
+
+    const { from } = view.state.selection.main
+    const line = view.state.doc.lineAt(from)
+    const currentLine = view.state.sliceDoc(line.from, line.to)
+
+    // 如果已经有前缀，移除它
+    if (currentLine.startsWith(prefix)) {
+      view.dispatch({
+        changes: { from: line.from, to: line.from + prefix.length, insert: '' }
+      })
+    } else {
+      view.dispatch({
+        changes: { from: line.from, to: line.from, insert: prefix }
+      })
+    }
+    view.focus()
+  }, [viewRef])
+
+  const insertBlock = useCallback((text: string) => {
+    const view = viewRef.current
+    if (!view) return
+
+    const { from } = view.state.selection.main
+    const line = view.state.doc.lineAt(from)
+    const insertPos = line.to + 1
+
+    view.dispatch({
+      changes: { from: insertPos, to: insertPos, insert: '\n' + text + '\n' },
+      selection: { anchor: insertPos + 1, head: insertPos + 1 + text.length }
+    })
+    view.focus()
+  }, [viewRef])
+
+  return (
+    <div className="editor-toolbar">
+      <button className="toolbar-btn-sm" onClick={() => insertFormatting('**', '**', '粗体')} title="粗体 (Ctrl+B)">
+        <strong>B</strong>
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertFormatting('*', '*', '斜体')} title="斜体 (Ctrl+I)">
+        <em>I</em>
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertFormatting('~~', '~~', '删除线')} title="删除线">
+        <s>S</s>
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertFormatting('`', '`', '代码')} title="行内代码">
+        {'</>'}
+      </button>
+      <div className="toolbar-divider-sm" />
+      <button className="toolbar-btn-sm" onClick={() => insertLinePrefix('# ')} title="标题">
+        H1
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertLinePrefix('## ')} title="二级标题">
+        H2
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertLinePrefix('### ')} title="三级标题">
+        H3
+      </button>
+      <div className="toolbar-divider-sm" />
+      <button className="toolbar-btn-sm" onClick={() => insertLinePrefix('- ')} title="无序列表">
+        •≡
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertLinePrefix('1. ')} title="有序列表">
+        1.
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertLinePrefix('> ')} title="引用">
+        ❝
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertBlock('```\n代码\n```')} title="代码块">
+        {'{ }'}
+      </button>
+      <div className="toolbar-divider-sm" />
+      <button className="toolbar-btn-sm" onClick={() => insertFormatting('[', '](url)', '链接文本')} title="链接">
+        🔗
+      </button>
+      <button className="toolbar-btn-sm" onClick={() => insertFormatting('![', '](url)', '图片描述')} title="图片">
+        🖼
+      </button>
+    </div>
+  )
+}