feat(posts): add public post markdown export action

Made-with: Cursor

app/pages/@[publicSlug]/posts/[postSlug].vue

@@ -1,6 +1,11 @@
 <script setup lang="ts">
 import { unwrapApiBody, type ApiResponse } from '../../../utils/http/factory'
 import { extractFrontMatterDesc } from '../../../utils/markdown-front-matter'
+import {
+  buildMarkdownExportFileName,
+  downloadMarkdownFile,
+  normalizeMarkdownImageUrls,
+} from '../../../utils/markdown-export'
 import { renderSafeMarkdown } from '../../../utils/render-markdown'
 import { formatOccurredOnDisplay, occurredOnToIsoAttr } from '../../../utils/timeline-datetime'
 import { useAuthSession } from '../../../composables/useAuthSession'
@@ -13,6 +18,7 @@ const route = useRoute()
 const publicSlug = computed(() => route.params.publicSlug as string)
 const postSlug = computed(() => route.params.postSlug as string)
 const { user, loggedIn } = useAuthSession()
+const toast = useToast()
 
 type Post = {
   id: number
@@ -75,6 +81,26 @@ const canEditPost = computed(() => {
 const editPostHref = computed(() =>
   data.value && canEditPost.value ? `/me/posts/${data.value.id}` : '',
 )
+
+function exportMarkdown(): void {
+  if (!data.value) {
+    return
+  }
+
+  try {
+    const origin = window.location.origin
+    const bodyMarkdown = data.value.bodyMarkdown || ''
+    const normalizedMarkdown = normalizeMarkdownImageUrls(bodyMarkdown, origin)
+    const filename = buildMarkdownExportFileName({
+      slug: data.value.slug,
+      id: data.value.id,
+    })
+    downloadMarkdownFile(filename, normalizedMarkdown)
+    toast.add({ title: '已导出 Markdown', color: 'success' })
+  } catch {
+    toast.add({ title: '导出失败，请稍后重试', color: 'error' })
+  }
+}
 </script>
 
 <template>
@@ -97,6 +123,16 @@ const editPostHref = computed(() =>
         >
           编辑
         </UButton>
+        <UButton
+          v-if="data"
+          color="neutral"
+          variant="soft"
+          size="sm"
+          :disabled="pending"
+          @click="exportMarkdown"
+        >
+          导出 .md
+        </UButton>
       </div>
       <div v-if="data.coverUrl" class="flex justify-center">
         <img