fix(web): scope task list item index to memo content container

The closest() selector was targeting a CSS class that never existed on
the container, causing fallback to document.body and collecting task
items across all visible memos. This caused index collisions when
multiple memos with todo lists were on the page.

Adds data-memo-content attribute to the container and updates the
selector accordingly.

Fixes #5635

web/src/components/MemoContent/TaskListItem.tsx

@@ -38,7 +38,7 @@ export const TaskListItem: React.FC<TaskListItemProps> = ({ checked, node: _node
       // Fallback: Calculate index by counting all task list items in the entire memo
       // We need to search from the root memo content container, not just the nearest list
       // to ensure nested tasks are counted in document order
-      let searchRoot = listItem.closest('[class*="memo-content"], [class*="MemoContent"]');
+      let searchRoot = listItem.closest("[data-memo-content]");
 
       // If memo content container not found, search from document body
       if (!searchRoot) {

web/src/components/MemoContent/index.tsx

@@ -38,6 +38,7 @@ const MemoContent = (props: MemoContentProps) => {
     <div className={`w-full flex flex-col justify-start items-start text-foreground ${className || ""}`}>
       <div
         ref={memoContentContainerRef}
+        data-memo-content
         className={cn(
           "relative w-full max-w-full wrap-break-word text-base leading-6",
           "[&>*:last-child]:mb-0",