refactor: 删除分屏功能 + 代码审计修复 + 安全加固

## 删除分屏功能
- 删除 ViewMode 'split' 类型，仅保留 editor/preview
- 删除 splitRatio 状态和 Resizer 组件
- 删除滚动同步模块 (scrollSync.ts, rehypeSourceLine.ts)
- 更新快捷键: Ctrl+1 编辑, Ctrl+2 预览

## Bug 修复
- 修复 Toast setTimeout 内存泄漏 (App.tsx)
- 修复 ModifiedBanner reload 更新错误标签 (改用 modifiedFilePath 匹配)
- 修复 FileWatcher error 未重置 isSelfWriting (file-watcher.ts)
- 修复另存为时未 stop watcher (ipc-handlers.ts)

## 死代码清理 (10 项)
- 删除 main/ipc-channels.ts (与 shared/ 重复)
- 删除 main/file-system.ts 未使用的 formatBytes
- 删除 constants.ts 6 个未使用常量
- 删除 fileUtils.ts 未使用的 formatBytes
- 删除 Icons.tsx 5 个未使用图标 (SplitView/ChevronUp/ChevronDown/X/ArrowUp/ArrowDown)
- 删除 useCodeMirror 未使用的 getContent/scrollTo
- 删除 TabBar 未使用的 menuRef
- 删除 Tab.scrollLeft/previewScrollTop 字段
- 删除 tabStore 未使用的 getTabIndex
- 删除 Toast/ModifiedBanner 多余 React import

## 安全加固
- ipc-handlers: 添加路径遍历防护 (validatePath 函数)
- preload: openExternal 仅允许 http/https 协议
- window-manager: 启用 sandbox: true
- preload: removeAllListeners 改为精确取消订阅 (返回 Unsubscribe 函数)

## 状态持久化
- activeTabId 持久化到 IndexedDB (刷新后恢复正确标签)
- sidebar 状态持久化到 IndexedDB (isVisible/sidebarWidth)

## 代码优化
- preload 使用 IPC_CHANNELS 常量替代硬编码字符串
- ipc-handlers _event 类型改为 IpcMainInvokeEvent
- settingsRepository 删除 splitRatio 字段

src/renderer/components/Editor/Editor.tsx

@@ -22,8 +22,7 @@ export function Editor({ darkMode }: EditorProps) {
     getScrollTop,
     setScrollTop,
     getSelection,
-    setSelection,
-    setLineAtTop
+    setSelection
   } = useCodeMirror({
     content: activeTab?.content ?? '',
     onChange: useCallback((value: string) => {
@@ -31,25 +30,9 @@ export function Editor({ darkMode }: EditorProps) {
       updateTabContent(activeTabId, value)
       setModified(activeTabId, true)
     }, [activeTabId, updateTabContent, setModified]),
-    darkMode,
-    onScroll: useCallback((line: number) => {
-      // 派发行号给预览侧做滚动同步
-      window.dispatchEvent(new CustomEvent('editor-scroll', { detail: { line } }))
-    }, [])
+    darkMode
   })
 
-  // 反向同步：监听预览滚动事件
-  useEffect(() => {
-    const handlePreviewScroll = (e: Event) => {
-      const line = (e as CustomEvent).detail?.line
-      if (typeof line === 'number') {
-        setLineAtTop(Math.floor(line) + 1) // setLineAtTop 是 1-indexed
-      }
-    }
-    window.addEventListener('preview-scroll', handlePreviewScroll)
-    return () => window.removeEventListener('preview-scroll', handlePreviewScroll)
-  }, [setLineAtTop])
-
   // 切换标签时加载内容
   useEffect(() => {
     if (!activeTab) return

src/renderer/components/Editor/useCodeMirror.ts

@@ -11,10 +11,9 @@ interface UseCodeMirrorOptions {
   content: string
   onChange: (value: string) => void
   darkMode: boolean
-  onScroll?: (line: number) => void
 }
 
-export function useCodeMirror({ content, onChange, darkMode, onScroll }: UseCodeMirrorOptions) {
+export function useCodeMirror({ content, onChange, darkMode }: UseCodeMirrorOptions) {
   const containerRef = useRef<HTMLDivElement>(null)
   const viewRef = useRef<EditorView | null>(null)
   const isExternalUpdate = useRef(false)
@@ -81,23 +80,7 @@ export function useCodeMirror({ content, onChange, darkMode, onScroll }: UseCode
 
     viewRef.current = view
 
-    // 滚动事件监听：派发行号（支持小数表示行内偏移）
-    const handleScroll = () => {
-      if (onScroll) {
-        const dom = view.scrollDOM
-        const scrollTop = dom.scrollTop
-        const block = view.lineBlockAtHeight(scrollTop)
-        const line = view.state.doc.lineAt(block.from)
-        const fraction = block.height > 0
-          ? (scrollTop - block.top) / block.height
-          : 0
-        onScroll(line.number - 1 + Math.max(0, fraction))  // 0-indexed + 行内比例
-      }
-    }
-    view.scrollDOM.addEventListener('scroll', handleScroll)
-
     return () => {
-      view.scrollDOM.removeEventListener('scroll', handleScroll)
       view.destroy()
       viewRef.current = null
     }
@@ -116,12 +99,6 @@ export function useCodeMirror({ content, onChange, darkMode, onScroll }: UseCode
       isExternalUpdate.current = false
     }
   }, [])
-
-  // 获取当前内容
-  const getContent = useCallback(() => {
-    return viewRef.current?.state.doc.toString() ?? ''
-  }, [])
-
   // 获取/设置滚动位置
   const getScrollTop = useCallback(() => {
     return viewRef.current?.scrollDOM.scrollTop ?? 0
@@ -147,35 +124,13 @@ export function useCodeMirror({ content, onChange, darkMode, onScroll }: UseCode
     view.dispatch({ selection: { anchor: from, head: to } })
     view.focus()
   }, [])
-
-  // 滚动到指定行
-  const scrollTo = useCallback((pos: number) => {
-    const view = viewRef.current
-    if (!view) return
-    view.dispatch({ effects: EditorView.scrollIntoView(pos, { y: 'center' }) })
-  }, [])
-
-  // 滚动使指定行出现在编辑器顶部（反向同步用）
-  const setLineAtTop = useCallback((lineNumber: number) => {
-    const view = viewRef.current
-    if (!view) return
-    const line = Math.max(1, Math.min(lineNumber, view.state.doc.lines))
-    const lineInfo = view.state.doc.line(line)
-    const block = view.lineBlockAt(lineInfo.from)
-    const currentTop = view.lineBlockAtHeight(view.scrollDOM.scrollTop).top
-    view.scrollDOM.scrollTop += block.top - currentTop
-  }, [])
-
   return {
     containerRef,
     viewRef,
     setContent,
-    getContent,
     getScrollTop,
     setScrollTop,
     getSelection,
-    setSelection,
-    scrollTo,
-    setLineAtTop
+    setSelection
   }
 }