#42831 refactor: share `useDeferredMenuMount` from ui-client

Proposed changes (including videos or screenshots)

Moves useDeferredMenuMount from apps/meteor/client/sidebar/Item to @rocket.chat/ui-client, so list rows outside the sidebar (room members, team channels) can defer mounting their menus the same way. No behavior change: the current sidebar callers import it from @rocket.chat/ui-client.

Issue(s)

Part of the Item migration. Needed by #42818 and #42819, which carry this commit at the bottom until it lands.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Refactor
  • Sidebar menus now use a shared deferred-mounting capability. The change does not alter how sidebar menus behave.
• Documentation
  • Clarified that a list-row menu mounts when focused.