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.

package.json

@@ -9,12 +9,17 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@milkdown/core": "^7.18.0",
+    "@milkdown/crepe": "^7.18.0",
+    "@milkdown/kit": "^7.18.0",
+    "@milkdown/theme-nord": "^7.18.0",
+    "@milkdown/vue": "^7.18.0",
     "axios": "^1.13.2",
-    "pinia": "^2.3.1",
-    "vue": "^3.5.24",
-    "vue-router": "^4.6.4",
     "markdown-it": "^13.0.0",
-    "prismjs": "^1.29.0"
+    "pinia": "^2.3.1",
+    "prismjs": "^1.29.0",
+    "vue": "^3.5.24",
+    "vue-router": "^4.6.4"
   },
   "devDependencies": {
     "@vitejs/plugin-vue": "^6.0.1",