ui-pagescene: hoist useViewportSize above early return in ControlPanelSceneObject

The hook was called below the `if (!currentSceneObject) return null`
guard, so renders where `currentSceneObject` was null skipped the hook
entirely. When it flipped to non-null on a later render, React saw a
new hook appear and threw "rendered more hooks than during the previous
render".

Move `useViewportSize()` (and the `getScale` helper that consumes it)
above the early return so the hook order is stable across renders.

frontend/libs/components/pagescene/src/lib/comps/ControlPanelSceneObject/ControlPanelSceneObject.tsx

@@ -212,6 +212,15 @@ export const ControlPanelSceneObject = () => {
     };
   }, [isShowing]);
 
+  // Must come before the early return below — hooks have to run in
+  // the same order every render or React errors with "rendered more
+  // hooks than during the previous render".
+  const viewport = useViewportSize();
+  const getScale = () => {
+    const height = viewport.height - 56;
+    return height < 620 ? height / 620 : 1;
+  };
+
   if (!currentSceneObject) {
     return null;
   }
@@ -276,12 +285,6 @@ export const ControlPanelSceneObject = () => {
     editorEngine?.deleteObject(currentSceneObject.object_uuid);
   };
 
-  const viewport = useViewportSize();
-  const getScale = () => {
-    const height = viewport.height - 56;
-    return height < 620 ? height / 620 : 1;
-  };
-
   return (
     <Transition
       as="div"