feat(editor): implement WYSIWYG Markdown editor using Milkdown Crepe

Replace the existing contenteditable-based markdown editor with a full-featured WYSIWYG editor using @milkdown/crepe. The new implementation provides:
- True WYSIWYG editing experience with instant Markdown syntax rendering
- Slash command menu support for quick formatting
- Code block highlighting and image paste support
- Built-in export to markdown file functionality

Changes include new MilkdownEditor component, updated App.vue integration, theme styling imports, and optimized Vite configuration for the new dependencies.

plans/milkdown-editor-plan.md

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+# Milkdown 全屏 WYSIWYG Markdown 编辑器实施方案
+
+## 项目概述
+
+基于 `@milkdown/crepe` 实现全屏覆盖的所见即所得 Markdown 编辑器，替换现有的 contenteditable 编辑器。
+
+## 技术选型
+
+- **核心编辑器**: `@milkdown/crepe` - 功能完整的 WYSIWYG Markdown 编辑器
+- **Vue 集成**: `@milkdown/vue` - Vue 3 组件支持
+- **主题**: `frame`（简洁框架主题）
+
+## 系统架构
+
+```mermaid
+graph TB
+    A[App.vue] --> B[MilkdownProvider]
+    B --> C[Milkdown Editor - Crepe]
+    
+    subgraph "Crepe 核心功能"
+        D[WYSWIYG 编辑体验]
+        E[Markdown 语法即时渲染]
+        F[斜杠命令菜单 Slash Commands]
+        G[代码块高亮]
+        H[图片粘贴支持]
+    end
+```
+
+## 实施步骤
+
+### Step 1: 安装依赖包
+
+```bash
+npm install @milkdown/crepe @milkdown/vue
+```
+
+### Step 2: 创建 Milkdown 编辑器组件
+
+**文件**: `src/components/MilkdownEditor.vue`
+
+```vue
+<template>
+  <MilkdownProvider>
+    <Milkdown />
+  </MilkdownProvider>
+</template>
+
+<script setup>
+import { Milkdown, MilkdownProvider, useEditor } from '@milkdown/vue'
+import { Crepe } from '@milkdown/crepe'
+
+const { get } = useEditor((root) => new Crepe({ root }))
+</script>
+```
+
+### Step 3: 更新 App.vue
+
+- 移除旧的 `MarkdownEditor` 组件引用
+- 使用新的 `MilkdownEditor` 组件
+- 保持全屏布局（100vh × 100vw）
+
+### Step 4: 添加样式配置
+
+在 `main.js` 中导入 Crepe 主题：
+```js
+import '@milkdown/crepe/theme/common/style.css'
+import '@milkdown/crepe/theme/frame.css'
+```
+
+## 全屏覆盖样式要点
+
+- 编辑器容器: `width: 100vw; height: 100vh`
+- 移除默认 padding/margin
+- 纯编辑器模式，无预览面板