refactor(web): 工作区布局 - 侧栏底部去分隔线改淡背景

- 侧栏 footer 移除 border-t 分隔线，改用淡背景（foreground 4%）+ 底部圆角匹配卡片，层级由明度差表达

web/src/components/layout/workspace-sidebar-nav.tsx

@@ -344,7 +344,7 @@ export function WorkspaceSidebarNav({ onNavigate, onOpenSearch }: { onNavigate:
                 ))}
             </div>
 
-            <div className="app-workspace-sidebar-footer shrink-0 border-t border-[var(--workspace-border)] px-3 py-3">
+            <div className="app-workspace-sidebar-footer shrink-0 rounded-b-[var(--r-xl)] bg-foreground/[.04] px-3 py-3">
                 <div className="flex flex-col gap-0.5">
                     {footer.map((item) => (
                         <NavItem