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/streetLevelMarkup.test.mjs

@@ -32,7 +32,7 @@ test('Street Level is a collapsible right-rail panel that starts collapsed', ()
   assert.doesNotMatch(html, /sl-mark|mly-mark/);
   assert.match(
     html,
-    /<div class="street-level-panel-inner" data-rail-scroller>/,
+    /<div class="street-level-panel-inner ui-scroll" data-rail-scroller>/,
   );
   assert.doesNotMatch(
     html,