feat(compose): 正文内嵌隐藏块并升级 Markdown 编辑器

去掉整帖可见性表单，改为 :::hide 按块脱敏；工具栏补齐代码块、链接/图片对话框与粘贴拖拽上传。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/postMeta.ts

@@ -15,6 +15,7 @@ export const CONTENT_ACCESS = [
   { value: "login", label: "登录可见" },
   { value: "reply", label: "回复可见" },
   { value: "points", label: "积分可见" },
+  { value: "mixed", label: "含隐藏内容" },
 ] as const;
 
 export type ContentAccessValue = (typeof CONTENT_ACCESS)[number]["value"];