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/stores/editorStore.ts

@@ -4,21 +4,17 @@ import type { ViewMode } from '../types/settings'
 interface EditorState {
   viewMode: ViewMode
   darkMode: boolean
-  splitRatio: number
 
   setViewMode: (mode: ViewMode) => void
   setDarkMode: (dark: boolean) => void
-  setSplitRatio: (ratio: number) => void
   toggleDarkMode: () => void
 }
 
 export const useEditorStore = create<EditorState>((set) => ({
-  viewMode: 'split',
+  viewMode: 'editor',
   darkMode: false,
-  splitRatio: 50,
 
   setViewMode: (mode) => set({ viewMode: mode }),
   setDarkMode: (dark) => set({ darkMode: dark }),
-  setSplitRatio: (ratio) => set({ splitRatio: ratio }),
   toggleDarkMode: () => set(state => ({ darkMode: !state.darkMode }))
 }))

src/renderer/stores/sidebarStore.ts

@@ -1,5 +1,6 @@
 import { create } from 'zustand'
 import type { FileNode } from '../types/file'
+import { settingsRepository } from '../db/settingsRepository'
 
 interface SidebarState {
   isVisible: boolean
@@ -7,6 +8,7 @@ interface SidebarState {
   tree: FileNode[]
   expandedDirs: Set<string>
   sidebarWidth: number
+  _loaded: boolean
 
   setVisible: (visible: boolean) => void
   setRootPath: (path: string | null) => void
@@ -14,16 +16,37 @@ interface SidebarState {
   toggleDir: (path: string) => void
   expandDirs: (paths: string[]) => void
   setSidebarWidth: (width: number) => void
+  loadFromDB: () => Promise<void>
 }
 
-export const useSidebarStore = create<SidebarState>((set) => ({
+export const useSidebarStore = create<SidebarState>((set, get) => ({
   isVisible: true,
   rootPath: null,
   tree: [],
   expandedDirs: new Set<string>(),
   sidebarWidth: 240,
+  _loaded: false,
 
-  setVisible: (visible) => set({ isVisible: visible }),
+  // 从 IndexedDB 加载 sidebar 设置
+  loadFromDB: async () => {
+    if (get()._loaded) return
+    try {
+      const settings = await settingsRepository.load()
+      set({
+        isVisible: !settings.sidebarCollapsed,
+        sidebarWidth: settings.sidebarWidth,
+        _loaded: true
+      })
+    } catch {
+      set({ _loaded: true })
+    }
+  },
+
+  setVisible: (visible) => {
+    set({ isVisible: visible })
+    // 异步保存到 DB
+    settingsRepository.save({ sidebarCollapsed: !visible })
+  },
   setRootPath: (path) => set({ rootPath: path }),
   setTree: (tree) => set({ tree }),
   toggleDir: (path) =>
@@ -48,5 +71,9 @@ export const useSidebarStore = create<SidebarState>((set) => ({
       }
       return changed ? { expandedDirs: newSet } : state
     }),
-  setSidebarWidth: (width) => set({ sidebarWidth: width })
+  setSidebarWidth: (width) => {
+    set({ sidebarWidth: width })
+    // 异步保存到 DB
+    settingsRepository.save({ sidebarWidth: width })
+  }
 }))

src/renderer/stores/tabStore.ts

@@ -18,8 +18,7 @@ interface TabState {
   updateTabContent: (tabId: string, content: string) => void
   setModified: (tabId: string, modified: boolean) => void
   getActiveTab: () => Tab | null
-  getTabIndex: (tabId: string) => number
-  updateTabScroll: (tabId: string, scroll: Partial<Pick<Tab, 'scrollTop' | 'scrollLeft' | 'selectionStart' | 'selectionEnd' | 'previewScrollTop'>>) => void
+  updateTabScroll: (tabId: string, scroll: Partial<Pick<Tab, 'scrollTop' | 'selectionStart' | 'selectionEnd'>>) => void
   loadFromDB: () => Promise<void>
   saveToDB: () => Promise<void>
 }
@@ -34,7 +33,10 @@ export const useTabStore = create<TabState>((set, get) => ({
   loadFromDB: async () => {
     if (get()._loaded) return
     try {
-      const snapshots = await tabRepository.loadAll()
+      const [snapshots, savedActiveTabId] = await Promise.all([
+        tabRepository.loadAll(),
+        tabRepository.loadActiveTabId()
+      ])
       if (snapshots.length > 0) {
         const tabs: Tab[] = snapshots.map(s => ({
           id: s.id,
@@ -42,14 +44,16 @@ export const useTabStore = create<TabState>((set, get) => ({
           content: s.content,
           isModified: s.isModified,
           scrollTop: s.scrollTop,
-          scrollLeft: s.scrollLeft,
           selectionStart: s.selectionStart,
-          selectionEnd: s.selectionEnd,
-          previewScrollTop: s.previewScrollTop
+          selectionEnd: s.selectionEnd
         }))
+        // 使用保存的 activeTabId，如果不存在则取最后一个
+        const activeTabId = (savedActiveTabId && tabs.find(t => t.id === savedActiveTabId))
+          ? savedActiveTabId
+          : tabs[tabs.length - 1].id
         set({
           tabs,
-          activeTabId: tabs[tabs.length - 1].id,
+          activeTabId,
           _loaded: true
         })
       } else {
@@ -73,13 +77,12 @@ export const useTabStore = create<TabState>((set, get) => ({
       content: t.content,
       isModified: t.isModified,
       scrollTop: t.scrollTop,
-      scrollLeft: t.scrollLeft,
       selectionStart: t.selectionStart,
       selectionEnd: t.selectionEnd,
-      previewScrollTop: t.previewScrollTop,
       updatedAt: Date.now()
     }))
     await tabRepository.saveAll(snapshots)
+    await tabRepository.saveActiveTabId(get().activeTabId)
   },
 
   createTab: (filePath = null, content = '') => {
@@ -97,10 +100,8 @@ export const useTabStore = create<TabState>((set, get) => ({
       content,
       isModified: false,
       scrollTop: 0,
-      scrollLeft: 0,
       selectionStart: 0,
-      selectionEnd: 0,
-      previewScrollTop: 0
+      selectionEnd: 0
     }
 
     set(state => ({
@@ -187,6 +188,8 @@ export const useTabStore = create<TabState>((set, get) => ({
         : state.mruStack
       return { activeTabId: tabId, mruStack: newMru }
     })
+    // 异步保存 activeTabId
+    setTimeout(() => tabRepository.saveActiveTabId(tabId), 0)
   },
 
   updateTabContent: (tabId, content) => {
@@ -210,10 +213,6 @@ export const useTabStore = create<TabState>((set, get) => ({
     return tabs.find(t => t.id === activeTabId) ?? null
   },
 
-  getTabIndex: (tabId) => {
-    return get().tabs.findIndex(t => t.id === tabId)
-  },
-
   updateTabScroll: (tabId, scroll) => {
     set(state => ({
       tabs: state.tabs.map(t =>