feat(admin): Markdown 编辑器分栏实时预览 + 工具栏

- 左写右预览，react-markdown + remark-gfm（表格/GFM 生效）
- 工具栏一键插入标题/加粗/斜体/代码/链接/列表/引用（按光标选区）
- 预览接 prose-site 排版，编辑页返回链接指向 /posts

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

admin/package.json

@@ -10,12 +10,15 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@tailwindcss/typography": "^0.5.20",
     "@tailwindcss/vite": "^4.3.3",
     "clsx": "^2.1.1",
     "lucide-react": "^1.24.0",
     "react": "^19.2.7",
     "react-dom": "^19.2.7",
+    "react-markdown": "^10.1.0",
     "react-router-dom": "^7.18.1",
+    "remark-gfm": "^4.0.1",
     "tailwind-merge": "^3.6.0",
     "tailwindcss": "^4.3.3"
   },