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/plugins/md-editor.client.ts

@@ -0,0 +1,6 @@
+import { MdEditor } from 'md-editor-v3'
+import 'md-editor-v3/lib/style.css'
+
+export default defineNuxtPlugin((nuxtApp) => {
+    nuxtApp.vueApp.component('MdEditor', MdEditor)
+})