PageScene: record color changes via HistoryManager

Color picker is the third interactive surface (after gizmo drag and
outliner toggles) that mutates an object. Wire it the same way:
record in the onChange handler (input type="color" fires once per
committed pick, so one undo entry per user choice).

Object Panel transform inputs (position/rotation/scale text fields)
are still on the legacy diff path. Wiring them is one more pass —
they need a coalescer because each keystroke flushes to the engine.
Tracking that as a follow-up.

frontend/apps/artcraft/app/src/pages/PageScene/comps/ControlPanelSceneObject/ControlPanelSceneObject.tsx

@@ -220,11 +220,17 @@ export const ControlPanelSceneObject = () => {
             className="h-0 w-0 cursor-pointer opacity-0"
             id={colorInputId}
             onChange={(e: ChangeEvent<HTMLInputElement>) => {
-              editorEngine?.activeScene.setColor(
-                editorEngine?.selected?.uuid || "",
-                e.target.value,
-              );
-              setColor(e.target.value);
+              // <input type="color"> fires onChange once per committed
+              // pick (when the picker closes), so recording here gives
+              // exactly one undo entry per user-visible color change.
+              const uuid = editorEngine?.selected?.uuid;
+              const before = color;
+              const after = e.target.value;
+              editorEngine?.activeScene.setColor(uuid || "", after);
+              if (uuid) {
+                editorEngine?.history.recordSetColor(uuid, before, after);
+              }
+              setColor(after);
             }}
             type="color"
             value={color}