feat(web): Hero 背景改用 AI 生成插画 (agnes-image-2.1-flash)

- 新增 web/img/hero-family-bg.png (1792x1024)
- 自然暖色调抽象家庭插画：屋顶/成长树/太阳/彩虹弧/家庭剪影
- 右侧留白用于文字叠加
- .hero-bg 更新为引用真实图片而非 SVG

web/css/style.css

@@ -428,7 +428,7 @@ ul { list-style: none; }
   position: absolute;
   inset: 0;
   background:
-    url(../images/hero-family.svg) center/cover no-repeat,
+    url(../img/hero-family-bg.png) center/cover no-repeat,
     radial-gradient(ellipse 80% 50% at 50% -10%, rgba(74, 155, 215, .10), transparent),
     radial-gradient(ellipse 60% 40% at 80% 80%, rgba(78, 205, 196, .08), transparent),
     radial-gradient(ellipse 50% 50% at 20% 70%, rgba(255, 217, 61, .06), transparent);