feat(web): draw the sidebar's scrollbar only under the pointer

The session list overflows after a handful of sessions, and its scrollbar
was drawn permanently in a column that is at rest most of the time.

SidebarRoot now tracks the pointer over the whole column and rebinds
ui-theme's scrollbar indirection pair to `transparent` while it is
outside, keeping the thumb for 2s after the pointer leaves so it does not
blink out on the way past. Rebinding colour leaves the list's
`scrollbar-gutter: stable` reservation in force, so revealing the bar
moves no row.

ui-theme's gate now states the widened contract: a rebind targets an -l2
token pair or `transparent`, and nothing else.

packages/client/ui-sidebar/README.zh.md

@@ -8,6 +8,8 @@ New Session 会启动运行时的页面局部前端 Session Intent；真实 Work
 
 `SidebarRootComponentProps` 组合布局 owner share、全局 `useSessions` 和 `useWorkspaces` 钩子、已声明的 `sidebar.workspace` 与 `sidebar.settings` 子 slot，以及注入的 `startSession`、`open` 和侧边栏切换回调。这里没有插件 store：`deriveGroups` 消费对象层快照与组件局部的展开／搜索状态。
 
+栏内的滚动条是一种指针可供性：只要指针不在栏内，外壳就把 ui-theme 的[滚动条间接层](../ui-theme/README.md)重新绑定为 `transparent`；指针离开后滑块再保留 2 秒，因此没人指向的列表不会带着滚动条。避免行位移的空间预留属于滚动区域本身（[ui-workspace](../ui-workspace/README.md)），所以显示滑块不会引起重排。
+
 页脚承载 `sidebar.settings`：侧边栏只渲染固定在底部的布局 slot，并共享其栏状态（`wide`）；ui-settings 在此注册触发行和设置面板。
 
 `/client` 导出表层只包含插件主体（`apply`／`inject`）及契约类型：SidebarRoot、行组件和树派生均属于内部实现（slot 注册通过闭包引用它们；测试直接导入 src 路径）。