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/stores/searchStore.ts

@@ -1,64 +0,0 @@
-import { create } from 'zustand'
-import type { SearchMatch, SearchOptions } from '../types/search'
-
-interface SearchState {
-  isVisible: boolean
-  showReplace: boolean
-  searchText: string
-  replaceText: string
-  matches: SearchMatch[]
-  currentIndex: number
-  options: SearchOptions
-
-  setVisible: (visible: boolean) => void
-  setShowReplace: (show: boolean) => void
-  setSearchText: (text: string) => void
-  setReplaceText: (text: string) => void
-  setMatches: (matches: SearchMatch[]) => void
-  setCurrentIndex: (index: number) => void
-  toggleCaseSensitive: () => void
-  toggleRegex: () => void
-  findNext: () => void
-  findPrev: () => void
-  close: () => void
-}
-
-export const useSearchStore = create<SearchState>((set, get) => ({
-  isVisible: false,
-  showReplace: false,
-  searchText: '',
-  replaceText: '',
-  matches: [],
-  currentIndex: -1,
-  options: { caseSensitive: false, useRegex: false },
-
-  setVisible: (visible) => set({ isVisible: visible }),
-  setShowReplace: (show) => set({ showReplace: show }),
-  setSearchText: (text) => set({ searchText: text }),
-  setReplaceText: (text) => set({ replaceText: text }),
-  setMatches: (matches) => set({ matches, currentIndex: matches.length > 0 ? 0 : -1 }),
-  setCurrentIndex: (index) => set({ currentIndex: index }),
-  toggleCaseSensitive: () =>
-    set(state => ({ options: { ...state.options, caseSensitive: !state.options.caseSensitive } })),
-  toggleRegex: () =>
-    set(state => ({ options: { ...state.options, useRegex: !state.options.useRegex } })),
-  findNext: () => {
-    const { matches, currentIndex } = get()
-    if (matches.length === 0) return
-    set({ currentIndex: (currentIndex + 1) % matches.length })
-  },
-  findPrev: () => {
-    const { matches, currentIndex } = get()
-    if (matches.length === 0) return
-    set({ currentIndex: (currentIndex - 1 + matches.length) % matches.length })
-  },
-  close: () =>
-    set({
-      isVisible: false,
-      showReplace: false,
-      searchText: '',
-      replaceText: '',
-      matches: [],
-      currentIndex: -1
-    })
-}))