feat(studio): animate flat inspector group expand/collapse

Adds a fast (120ms) CSS entrance animation for flat inspector accordion
group headers/body, gated to the group actually toggling (not derived
from remounting alone) to avoid a Chromium reflow quirk that otherwise
replays the animation on untouched collapsed siblings.

packages/studio/src/styles/studio.css

@@ -447,3 +447,25 @@ body {
     animation: none;
   }
 }
+
+/* Flat inspector group header/body entrance (FlatGroupHeader) */
+@keyframes hf-flat-group-in {
+  from {
+    opacity: 0;
+    transform: translateY(-4px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+.hf-flat-group-enter {
+  animation: hf-flat-group-in 120ms ease-out;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .hf-flat-group-enter {
+    animation: none;
+  }
+}