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.

package.json

@@ -21,6 +21,7 @@
     "element-plus": "^2.11.4",
     "html2pdf.js": "^0.12.1",
     "maska": "^3.2.0",
+    "md-editor-v3": "^6.0.1",
     "nuxt": "^4.1.3",
     "reka-ui": "^2.5.1",
     "tailwindcss": "^4.1.14",