feat(pets): replace static cats with Live2D LittleCat model (black texture)

- Copied LittleCat model files to frontend/public/live2d/models/little-cat/
- Using the black alternate texture as default
- Created Live2DCat component that renders the model in a small canvas
- PetZone now shows a single Live2D cat instead of two SVG cats

frontend/src/components/Live2DCat.tsx

@@ -0,0 +1,87 @@
+import { useEffect, useRef } from 'react';
+
+interface Props {
+  className?: string;
+}
+
+export default function Live2DCat({ className }: Props) {
+  const canvasRef = useRef<HTMLCanvasElement>(null);
+
+  useEffect(() => {
+    let mounted = true;
+    const canvas = canvasRef.current;
+    if (!canvas) return;
+
+    let app: any = null;
+
+    const init = async () => {
+      try {
+        await loadScript('/live2d/cubism/live2dcubismcore.min.js');
+        const { Application, Ticker } = await import('pixi.js');
+        const { Live2DModel } = await import('pixi-live2d-display/cubism4');
+        (Live2DModel as any).registerTicker(Ticker as any);
+
+        const w = canvas.clientWidth || 120;
+        const h = canvas.clientHeight || 120;
+
+        app = new Application({
+          view: canvas,
+          width: w,
+          height: h,
+          antialias: true,
+          resolution: Math.min(window.devicePixelRatio || 1, 2),
+          backgroundAlpha: 0,
+          autoDensity: true,
+        });
+        if (!mounted) { app.destroy(true); return; }
+
+        const model = await Live2DModel.from('/live2d/models/little-cat/LittleCat.model3.json', {
+          autoInteract: false,
+        });
+        if (!mounted) { app.destroy(true); return; }
+
+        // Scale to fit the small canvas
+        const sw = app.screen.width;
+        const sh = app.screen.height;
+        const s = Math.min((sw * 0.85) / model.width, (sh * 0.85) / model.height);
+        model.scale.set(s);
+        model.anchor.set(0.5, 0.5);
+        model.position.set(sw / 2, sh / 2);
+
+        app.stage.addChild(model as any);
+        (model as any).isInteractive = () => false;
+
+        try { model.motion('Idle'); } catch { /* */ }
+
+      } catch (e) {
+        console.warn('[Live2DCat]', e);
+      }
+    };
+
+    init();
+
+    return () => {
+      mounted = false;
+      if (app) { app.destroy(true, { children: true }); }
+    };
+  }, []);
+
+  return (
+    <canvas
+      ref={canvasRef}
+      className={className}
+      style={{ display: 'block' }}
+    />
+  );
+}
+
+function loadScript(src: string): Promise<void> {
+  return new Promise((resolve, reject) => {
+    if (document.querySelector('script[src="' + src + '"]')) { resolve(); return; }
+    const s = document.createElement('script');
+    s.src = src;
+    s.onload = () => resolve();
+    s.onerror = () => reject(new Error('Failed ' + src));
+    document.head.appendChild(s);
+  });
+}