release: v0.4.0 — MetonaEditor 集成与架构重构

feat(editor): 将 Milkdown/ProseMirror 替换为 @metona-team/metona-editor v0.1.3
  - 三模式视图 (edit/split/preview) 由编辑器内置工具栏切换
  - searchReplace + imagePaste 预设插件
  - unified/rehype 渲染管线通过 render 钩子集成
  - 主题双向同步 (应用暗色模式 ↔ 编辑器主题)

feat(toast): metona-toast 迁移为 @metona-team/metona-toast v2.0.1

refactor: 删除冗余组件与代码
  - 移除 SourceEditor、Preview、SearchReplace、EditorToolbar、useMilkdown
  - 移除 StatusBar 组件及状态栏扩展架构（编辑器内置底栏替代）
  - 移除 useSettings、useStatusBarItem、useStatusBarItems、statusBarStore
  - 移除 EditMode/PreviewMode/SourceMode 图标

refactor(ui): 简化布局
  - 工具栏移除模式切换按钮，新增自动保存开关
  - useKeyboard 移除 Ctrl+1/2/3/B/I 快捷键
  - Editor 三模式统一由 MetonaEditor 容器渲染

chore: 版本号 v0.3.13 → v0.4.0
docs: 全面更新 README/DESIGN/CONTRIBUTING/DEVSETUP

package.json

@@ -1,6 +1,6 @@
 {
   "name": "marklite",
-  "version": "0.3.13",
+  "version": "0.4.0",
   "description": "Lightweight Markdown Editor for Windows",
   "main": "./dist/main/index.js",
   "scripts": {
@@ -28,20 +28,9 @@
   "author": "MarkLite",
   "license": "MIT",
   "dependencies": {
-    "@milkdown/core": "7.21.1",
-    "@milkdown/ctx": "7.21.1",
-    "@milkdown/plugin-clipboard": "7.21.1",
-    "@milkdown/plugin-history": "7.21.1",
-    "@milkdown/plugin-indent": "7.21.1",
-    "@milkdown/plugin-listener": "7.21.1",
-    "@milkdown/plugin-trailing": "7.21.1",
-    "@milkdown/preset-commonmark": "7.21.1",
-    "@milkdown/preset-gfm": "7.21.1",
-    "@milkdown/prose": "7.21.1",
-    "@milkdown/react": "7.21.1",
-    "@milkdown/utils": "7.21.1",
+    "@metona-team/metona-editor": "^0.1.3",
     "dexie": "^4.0.11",
-    "metona-toast": "file:../MetonaToast",
+    "@metona-team/metona-toast": "^2.0.1",
     "nanoid": "^5.1.5",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",