fix(pets): cat renders on shared KiraAvatar canvas via onAppReady callback

Single WebGL context, no bindBuffer spam. Cat model loads onto
KiraAvatar's stage and positions itself at bottom-right corner.
PetZone passes app prop through to Live2DCat.

frontend/src/components/KiraAvatar.tsx

@@ -7,6 +7,7 @@ interface Props {
   outfit: string;
   accessory: string | null;
   onTalkToggle: () => void;
+  onAppReady?: (app: any) => void;
 }
 
 type ExpressionName = 'Normal' | 'Smile' | 'Sad' | 'Angry' | 'Surprised' | 'Blushing';
@@ -82,6 +83,7 @@ export default function KiraAvatar(props: Props) {
         app.stage.addChild(model as any);
         (model as any).isInteractive = () => false;
         if (!mounted) return;
+        if (props.onAppReady) props.onAppReady(app);
 
         try {
           textureRef.current = {