Add 8px below the activity summary to separate it from new conversation. Remove redundant conversation-row arrows while keeping project submenu arrows, and update the UI documentation.
Validation: 11 ProjectActivity tests and typecheck passed. Lint was interrupted before completion.
Release note: 调整项目活动摘要与新建会话的间距,移除会话列表末尾的多余箭头,保留项目子菜单指示。