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/components/MilkdownEditor.vue

@@ -0,0 +1,97 @@
+<template>
+  <div class="editor-container">
+    <button class="export-btn" @click="exportMarkdown">导出文件</button>
+    <div ref="root" class="milkdown-editor"></div>
+  </div>
+</template>
+
+<script setup>
+import { onMounted, ref } from 'vue'
+import { Crepe } from '@milkdown/crepe'
+
+const root = ref(null)
+let crepe = null
+
+onMounted(async () => {
+  if (!root.value) return
+  
+  crepe = new Crepe({
+    root: root.value,
+    defaultValue: '# Welcome to Milkdown\n\nStart writing your markdown content here...',
+  })
+  
+  await crepe.create()
+})
+
+const exportMarkdown = async () => {
+  if (!crepe) return
+  const markdown = await crepe.getMarkdown()
+  const blob = new Blob([markdown], { type: 'text/markdown' })
+  const url = URL.createObjectURL(blob)
+  const a = document.createElement('a')
+  a.href = url
+  a.download = `document-${Date.now()}.md`
+  a.click()
+  URL.revokeObjectURL(url)
+}
+</script>
+
+<style scoped>
+.editor-container {
+  position: relative;
+}
+
+.export-btn {
+  position: fixed;
+  top: 20px;
+  right: 20px;
+  padding: 8px 16px;
+  background-color: #4a90d9;
+  color: white;
+  border: none;
+  border-radius: 4px;
+  cursor: pointer;
+  z-index: 1000;
+}
+
+.export-btn:hover {
+  background-color: #3a7bc8;
+}
+
+.milkdown-editor {
+  width: 100vw;
+  height: 100vh;
+  background-color: #ffffff;
+  overflow-y: auto;
+}
+
+/* 当内容不超过视口时隐藏滚动条 */
+.milkdown-editor::-webkit-scrollbar {
+  width: 8px;
+}
+
+.milkdown-editor::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+.milkdown-editor::-webkit-scrollbar-thumb {
+  background-color: #ddd;
+  border-radius: 4px;
+}
+
+/* 编辑器内容容器样式 */
+.milkdown-editor :deep(.milkdown) {
+  max-width: 900px;
+  margin: 0 auto !important;
+  padding: 20px 40px !important;
+  min-height: calc(100vh - 40px);
+}
+
+/* 全局覆盖所有元素的边距 */
+.milkdown-editor :deep(*) {
+  margin-top: 0 !important;
+  margin-bottom: 0 !important;
+  padding-top: 0 !important;
+  padding-bottom: 0 !important;
+}
+</style>