feat(ui): give every panel one scrollbar, select and slider skin

Every scrolling surface carries the kit's .ui-scroll, which
draws the thin themed scrollbar from --scroll-thumb and --scroll-track; the
Context panel and others drew the platform's light scrollbar because the
page declares no dark color-scheme. The per-panel scrollbar rules go; Cyber
sets the two tokens and Cockpit's HUD windows keep their accent tint.

Selects draw .gev-field's own chevron instead of the platform arrow, the
voice selects join .gev-field, number fields keep dark spinners, and the
Street Level range uses the shared slider in its green.

src/ui/localePicker.js

@@ -431,7 +431,7 @@ export function mountLocalePicker({
   heading.textContent = t('Interface language');
   popup.append(heading);
 
-  const listbox = element(documentRef, 'ul', 'locale-picker-list');
+  const listbox = element(documentRef, 'ul', 'locale-picker-list ui-scroll');
   listbox.id = ids.listbox;
   listbox.setAttribute('role', 'listbox');
   listbox.setAttribute('aria-labelledby', ids.heading);