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.md

@@ -4,6 +4,10 @@ English | [中文](README.zh.md)
 
 Theme plugin: ThemeService over the --dsw-* token base stylesheets (static scale + alias semantic layers). The service owns the theme preference (`light`/`dark`/`system`, persisted under `dsh.theme`), resolves `system` through `prefers-color-scheme`, and publishes immutable `ThemeSnapshot`s on the `theme/change` event; it never touches the DOM — ui-layout's presenter applies the resolved snapshot (`html { color-scheme }`, `body[data-ds-dark-theme]`, and inline alias tokens). Contract: api-contracts v3 §8.
 
+`src/styles/` holds five sheets, all imported by the web shell's `base.css`: `base.css`, `design-platform.css`, `scrollbar.css`, `gradient-shadow-text.css`, and `shiki.css`. `scrollbar.css` is the sole consumer of the `--dsw-alias-scrollbar-*` tokens and must follow `design-platform.css`, which declares them.
+
+Scrollbar rebinding contract: `scrollbar.css` binds `--dsh-scrollbar-thumb` and `--dsh-scrollbar-thumb-hover` on `body` to the l1 (base-surface) tokens, and both the standard `scrollbar-color` and the `::-webkit-scrollbar-thumb` rules read that pair. An elevated surface (menu, popover, dialog) sets `--dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2)` and `--dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2)` on its own container; one rebind retints both renderings. Reasoning and the measured computed values: [the scrollbar Agent Note](../../../.agents/notes/implemented/bug-fix/2026-07-28-themed-scrollbars-and-reserved-gutter.md).
+
 ## Model Experience
 
 None, as the theme service manages a browser preference; nothing here reaches a model request.