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.

src/main.js

@@ -2,4 +2,7 @@ import { createApp } from 'vue'
 import App from './App.vue'
 import './style.css'
 
+import '@milkdown/crepe/theme/common/style.css'
+import '@milkdown/crepe/theme/frame.css'
+
 createApp(App).mount('#app')