style(studio): distinguish open-section body from header background

Add panel.bg-inset token (#121214) and inset shadow to open-group body
for visual separation from the header row.

packages/studio/src/components/editor/PropertyPanelFlat.tsx

@@ -537,7 +537,7 @@ export function PropertyPanelFlat({
               animateEntrance={justToggledIds.includes(openGroup.id)}
             />
             <div
-              className={`${justToggledIds.includes(openGroup.id) ? "hf-flat-group-enter " : ""}min-h-0 flex-1 overflow-y-auto border-b border-panel-hairline px-4 py-3`}
+              className={`${justToggledIds.includes(openGroup.id) ? "hf-flat-group-enter " : ""}min-h-0 flex-1 overflow-y-auto border-b border-panel-hairline bg-panel-bg-inset px-4 py-3 shadow-[inset_0_2px_4px_-1px_rgba(0,0,0,0.5)]`}
             >
               {openGroup.content}
             </div>

packages/studio/src/styles/tailwind-preset.shared.js

@@ -12,6 +12,9 @@ const studioPreset = {
         },
         panel: {
           bg: "#0C0C0E",
+          // Open inspector-section body — slightly lighter than headers (bg)
+          // so the recessed scrollable region reads distinct.
+          "bg-inset": "#121214",
           input: "#161618",
           surface: "#18181B",
           hover: "#27272A",