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.

vite.config.js

@@ -1,7 +1,17 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
 
-// https://vite.dev/config/
 export default defineConfig({
   plugins: [vue()],
+  server: {
+    host: true,
+    port: 5173
+  },
+  optimizeDeps: {
+    include: [
+      '@milkdown/crepe',
+      '@milkdown/vue',
+      '@milkdown/kit'
+    ]
+  }
 })