fix(studio): pin user-agent color scheme to dark (heygen-com#482)

The studio shell paints a #0a0a0a body but never declares `color-scheme`,
so browsers render native UA chrome (scrollbars, form controls, focus
rings) in the light palette and the mismatch is obvious — especially
scrollbars, which appear as light tracks sitting on top of a near-black
panel.

Studio doesn't expose a theme toggle; the UI is dark-only. Declaring
`color-scheme: dark` on `:root` lines up the browser-native surfaces
with the rest of the chrome.

Author: MuTsunTsai

packages/studio/src/styles/studio.css

@@ -2,6 +2,15 @@
 @tailwind components;
 @tailwind utilities;
 
+/*
+ * Studio is a dark-only UI — pin the user-agent color scheme to dark so that
+ * browser-native chrome (scrollbars, form controls, focus rings) picks the
+ * matching palette instead of defaulting to light against our #0a0a0a body.
+ */
+:root {
+  color-scheme: dark;
+}
+
 body {
   margin: 0;
   padding: 0;