fix: refine project activity spacing and conversation rows

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。

src/renderer/src/ProjectActivity.tsx

@@ -209,7 +209,7 @@ function ActivityMenu({
             <small className={`project-activity__${activity.status === 'running' ? 'running' : 'attention'}`}>
               {t(`projectActivity.status.${activity.status}`)}
             </small>
-          </span><ChevronRight aria-hidden="true" size={14} />
+          </span>
         </button>)}
       </div>}
     </div>,

src/renderer/src/project-activity.css

@@ -21,6 +21,7 @@
   display: grid;
   grid-template-columns: minmax(0, 1fr) auto;
   flex-shrink: 0;
+  margin-bottom: var(--space-2);
   row-gap: var(--space-1);
   font-size: var(--font-caption);
 }