fix(ui-theme): gate the standard scrollbar properties behind the missing WebKit pseudo-element

A non-`auto` `scrollbar-width` or `scrollbar-color` makes Chromium and Safari
discard every `::-webkit-scrollbar*` rule for that element, including
`::-webkit-scrollbar-thumb:hover`. Declaring both unconditionally left the
hover tokens rendering nowhere: the engines implementing the hover
pseudo-element are exactly the ones the standard properties silence, and
Firefox has no hover pseudo-element to fall back on. Both hover tokens and all
four elevated surfaces' hover rebinds were therefore dead code.

Measured in chromium on probe elements with `scrollbar-gutter: stable`: an 8px
`::-webkit-scrollbar` alone reserved a 30px band, and adding
`scrollbar-width: thin` dropped it to the 10px `thin` reserves.

The standard properties now sit inside `@supports not
selector(::-webkit-scrollbar)`, so Firefox takes them and WebKit-based engines
take the pseudo-elements. The WebKit rules stay ungated: an engine without
those pseudo-elements drops them as unknown selectors, and gating them would
hide them from an engine that implements them without `selector()` — the
pre-16.4 Safari the ungated form serves correctly.

Three unit assertions pin the split by source offset, which the existing
at-rule-flattening parser cannot see. The web e2e now reads the path chromium
actually takes: the `auto` standard properties as the gate's signature, the
pseudo-element sizing and track, the indirection variables resolved per
throwaway probe, and the hover declaration as cascade rule text — chromium
folds the `:hover` rule into `getComputedStyle(el,
'::-webkit-scrollbar-thumb')`, so no computed query separates the states.

packages/client/ui-theme/README.md

@@ -6,7 +6,9 @@ Theme plugin: ThemeService over the --dsw-* token base stylesheets (static scale
 
 `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).
+Scrollbar rebinding contract: `scrollbar.css` binds `--dsh-scrollbar-thumb` and `--dsh-scrollbar-thumb-hover` on `body` to the l1 (base-surface) tokens, and both rendering paths 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 whichever path the engine took.
+
+The two paths are mutually exclusive by construction. `scrollbar-width`/`scrollbar-color` sit inside `@supports not selector(::-webkit-scrollbar)` because a non-`auto` value of either makes Chromium and Safari discard every `::-webkit-scrollbar*` rule for that element, `::-webkit-scrollbar-thumb:hover` included — declaring both unconditionally leaves `--dsh-scrollbar-thumb-hover` with no rendering anywhere. Firefox therefore takes the standard properties and WebKit-based engines take the pseudo-elements, so the hover token only ever renders through the pseudo-element path. 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