refactor: 删除自定义搜索，统一使用 CodeMirror 6 搜索

删除文件：
- SearchBar/SearchBar.tsx — 自定义搜索栏组件
- stores/searchStore.ts — 自定义搜索状态管理
- lib/searchEngine.ts — 自定义搜索匹配引擎
- types/search.ts — 搜索相关类型定义

清理代码：
- App.tsx：移除 SearchBar 导入和渲染
- useKeyboard.ts：移除 searchStore 引用和搜索快捷键
- Editor.tsx：移除 searchStore 引用和搜索逻辑
- types/index.ts：移除 search 类型导出
- global.css：删除旧搜索栏样式（~113 行）

CodeMirror 6 搜索面板移到顶部：
- .cm-panels { top: 0; bottom: auto; }
- .cm-panels-bottom { top: 0; bottom: auto; }

快捷键（CodeMirror 内置）：
- Ctrl+F 搜索
- Ctrl+H 替换
- Enter/Shift+Enter 下一个/上一个
- Alt+C 大小写
- Alt+R 正则

src/renderer/App.tsx

@@ -1,25 +1,23 @@
 import React, { useState, useCallback, useEffect, useRef } from 'react'
 import { useTabStore } from './stores/tabStore'
 import { useEditorStore } from './stores/editorStore'
-import { useSearchStore } from './stores/searchStore'
 import { useTheme } from './hooks/useTheme'
 import { useSettings } from './hooks/useSettings'
 import { useKeyboard } from './hooks/useKeyboard'
 import { useUnsavedWarning } from './hooks/useUnsavedWarning'
 import { useFileWatch } from './hooks/useFileWatch'
 import { recentFilesRepository } from './db/recentFilesRepository'
+import { useDragDrop } from './hooks/useDragDrop'
 import { Toolbar } from './components/Toolbar/Toolbar'
 import { TabBar } from './components/TabBar/TabBar'
 import { Editor } from './components/Editor/Editor'
 import { Preview } from './components/Preview/Preview'
 import { Sidebar } from './components/Sidebar/Sidebar'
-import { SearchBar } from './components/SearchBar/SearchBar'
 import { StatusBar } from './components/StatusBar/StatusBar'
 import { WelcomeScreen } from './components/WelcomeScreen/WelcomeScreen'
 import { Toast } from './components/Toast/Toast'
 import { ModifiedBanner } from './components/ModifiedBanner/ModifiedBanner'
 import { DropOverlay } from './components/DropOverlay/DropOverlay'
-import { useDragDrop } from './hooks/useDragDrop'
 
 export default function App() {
   const tabs = useTabStore(s => s.tabs)
@@ -29,7 +27,6 @@ export default function App() {
   const setModified = useTabStore(s => s.setModified)
   const updateTabContent = useTabStore(s => s.updateTabContent)
   const loadFromDB = useTabStore(s => s.loadFromDB)
-  const saveToDB = useTabStore(s => s.saveToDB)
 
   const viewMode = useEditorStore(s => s.viewMode)
   const splitRatio = useEditorStore(s => s.splitRatio)
@@ -80,9 +77,7 @@ export default function App() {
       const result = await window.electronAPI.openFile()
       if (result && 'filePath' in result) {
         createTab(result.filePath, result.content)
-        // 追踪最近打开文件
         if (result.filePath) recentFilesRepository.add(result.filePath)
-        // 保存标签页到 DB
         setTimeout(() => useTabStore.getState().saveToDB(), 100)
       }
     }
@@ -137,7 +132,7 @@ export default function App() {
     saveViewMode(mode)
   }, [saveViewMode])
 
-  // M-11: 主进程事件注册（使用 ref 保持最新回调引用 + cleanup 清理）
+  // M-11: 主进程事件注册
   const handleSaveRef = useRef(handleSave)
   const handleSaveAsRef = useRef(handleSaveAs)
   handleSaveRef.current = handleSave
@@ -149,7 +144,7 @@ export default function App() {
 
     const onOpen = (data: { filePath: string; content: string }) => {
       createTab(data.filePath, data.content)
-      recentFilesRepository.add(data.filePath)
+      if (data.filePath) recentFilesRepository.add(data.filePath)
       setTimeout(() => useTabStore.getState().saveToDB(), 100)
     }
     const onSave = () => handleSaveRef.current()
@@ -218,7 +213,6 @@ export default function App() {
             <div id="content-wrapper">
               {viewMode !== 'preview' && (
                 <div id="editor-panel" style={editorStyle}>
-                  <SearchBar />
                   <Editor darkMode={darkMode} />
                 </div>
               )}
@@ -270,7 +264,6 @@ function Resizer({ onResize }: { onResize: (ratio: number) => void }) {
       document.removeEventListener('mouseup', handleMouseUp)
       document.body.style.cursor = ''
       document.body.style.userSelect = ''
-      // 保存最终比例
       const wrapper = document.getElementById('content-wrapper')
       if (wrapper) {
         const rect = wrapper.getBoundingClientRect()