feat(admin): integrate markdown editor for news creation

This commit replaces the basic textarea in the 'Add News' modal with a full-featured Markdown editor.

- Adds the `md-editor-v3` dependency.
- Implements the editor within a `ClientOnly` component for client-side rendering.
- Creates a Nuxt plugin to register the `MdEditor` component globally.
- Adjusts the modal to be fullscreen to provide a better user experience for content creation.

app/components/admin/contents/news/AddModal.vue

@@ -1,29 +1,36 @@
 <template>
-  <UModal v-model:open="open" title="撰写新闻" description="在数据库中添加一篇新闻">
+  <UModal v-model:open="open" title="撰写新闻" description="在数据库中添加一篇新闻" fullscreen :ui="{
+    body: 'overflow-y-auto p-6', // 防止溢出 + 内边距
+    footer: 'justify-end'
+  }">
     <UButton label="撰写新闻" icon="mdi:newspaper-plus" />
 
     <template #body>
-      <UForm :schema="newsSchema" :state="newsState" @submit="onSubmit">
+      <UForm ref="form" :schema="newsSchema" :state="newsState" @submit="onSubmit" class="space-y-4">
         <UFormField label="新闻标题" name="title">
           <UInput v-model="newsState.title" placeholder="请输入新闻标题" class="w-full" />
         </UFormField>
         <UFormField label="新闻内容" name="contents">
-          <UTextarea v-model="newsState.content" class="w-full" />
+          <ClientOnly>
+            <MdEditor v-model="newsState.content" />
+          </ClientOnly>
         </UFormField>
-        这里应该是要有一个 Markdown 编辑界面
-        <div class="flex justify-end gap-2">
-          <UButton label="Cancel" color="neutral" variant="subtle" @click="open = false" />
-          <UButton label="Create" color="primary" variant="solid" type="submit" />
-        </div>
       </UForm>
     </template>
+
+    <template #footer>
+      <UButton label="Cancel" color="neutral" variant="subtle" @click="open = false" />
+      <UButton label="Create" color="primary" variant="solid" @click="form.submit()" />
+    </template>
   </UModal>
 </template>
 
 <script lang="ts" setup>
 import type { FormSubmitEvent } from '@nuxt/ui';
+import { MdEditor } from 'md-editor-v3';
 import * as z from 'zod'
 
+const form = ref();
 const open = ref(false);
 
 const newsSchema = z.object({