fix(ios): mobile modal occlusion issues (#11427)

Close [BS-2925](https://linear.app/affine-design/issue/BS-2925/移动端-dialog-遮挡相关)

blocksuite/affine/components/src/context-menu/menu-renderer.ts

@@ -490,6 +490,7 @@ export type MenuHandler = {
 
 const popMobileMenu = (options: MenuOptions): MenuHandler => {
   const model = createModal(document.body);
+  model.style.position = 'fixed';
   const menu = new Menu({
     ...options,
     onClose: () => {