fix(client): theme the scrollbars and reserve the workspace list gutter

design-platform.css declared four --dsw-alias-scrollbar-* tokens in both
palettes that no rule read, so every scrolling region rendered the user
agent's own scrollbar and the dark theme showed a light native bar against
dark surfaces.

The symptom that surfaced the gap was in the sidebar: the workspace
browser's session list is its only scrolling region, and each row's
trailing content (the relative timestamp, and the hover action buttons
that replace it) is `flex: none` flush against the row's 8px right
padding, so an overlaid scrollbar painted on top of the timestamp.

ui-theme/styles/scrollbar.css becomes the sole consumer of the four
tokens, imported by the web shell's base.css after design-platform.css
because it reads that sheet's tokens. The rules sit on `body`, not
`html`: the alias tokens are declared on `body`, custom properties
inherit only downward, and from `html` they resolve to the
guaranteed-invalid value with scrollbar-color computing to `auto`.
scrollbar-width and scrollbar-color are declared on `body, body *` rather
than inherited, because inheritance would carry the color already
substituted at `body` and an elevated surface could not retint its own
thumb; scrollbar-width does not inherit at all.

Both the standard properties and the ::-webkit-scrollbar pseudo-elements
read one indirection pair bound to the l1 tokens, so an elevated surface
rebinds that pair to the l2 tokens once and retints both renderings. The
command popup, slash menu, model-select panel, and settings panel do so,
which gives the l2 tokens their first consumers.

WorkspaceBrowser's `.list` declares scrollbar-gutter: stable, keeping the
bar beside the rows. `stable` rather than `auto` so the reservation holds
when the list is short enough not to scroll: expanding a workspace group
would otherwise shift every row sideways at the moment it starts
scrolling.

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

@@ -4,6 +4,10 @@
 
 主题插件：基于 --dsw-* token 基础样式表（静态尺度 + 别名语义层）的 ThemeService。该服务拥有主题偏好（`light`／`dark`／`system`，以 `dsh.theme` 为键持久化），将 `system` 通过 `prefers-color-scheme` 解析为实际主题，并发布不可变的 `ThemeSnapshot`，通过 `theme/change` 事件通知变化；它绝不接触 DOM：ui-layout 的呈现器会应用解析后的快照（`html { color-scheme }`、`body[data-ds-dark-theme]`，以及主题的别名 token 内联变量）。契约：api-contracts v3 §8。
 
+`src/styles/` 下有五张样式表，全部由 web 壳的 `base.css` 导入：`base.css`、`design-platform.css`、`scrollbar.css`、`gradient-shadow-text.css` 与 `shiki.css`。`scrollbar.css` 是 `--dsw-alias-scrollbar-*` token 的唯一消费方，必须排在声明这些 token 的 `design-platform.css` 之后。
+
+滚动条重新绑定契约：`scrollbar.css` 在 `body` 上把 `--dsh-scrollbar-thumb` 与 `--dsh-scrollbar-thumb-hover` 绑定到 l1（基础表面）token，标准属性 `scrollbar-color` 与 `::-webkit-scrollbar-thumb` 规则都读取这一组变量。抬升表面（菜单、浮层、对话框）在自己的容器上设置 `--dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2)` 与 `--dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2)`；一次重新绑定即可为两种渲染同时换色。推理过程与实测计算值见[滚动条 Agent Note](../../../.agents/notes/implemented/bug-fix/2026-07-28-themed-scrollbars-and-reserved-gutter.md)。
+
 ## 模型体验
 
 无。主题服务管理浏览器偏好；这里没有任何内容进入模型请求。