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/styles/provider-settings.css

@@ -184,19 +184,9 @@
   min-height: 0;
   overflow-y: auto;
   overscroll-behavior: contain;
-  scrollbar-width: thin;
-  scrollbar-color: rgba(54, 220, 255, 0.45) transparent;
   scrollbar-gutter: stable;
 }
 
-.key-setup-rows::-webkit-scrollbar {
-  width: 6px;
-}
-.key-setup-rows::-webkit-scrollbar-thumb {
-  background: rgba(54, 220, 255, 0.45);
-  border-radius: 3px;
-}
-
 .key-setup-row {
   display: grid;
   gap: 0.35rem;