fix: align website branding and spacing

The website used redrawn brand marks, generated icons retained opaque white edges, and adjacent download and Runtime sections accumulated excessive vertical space. It now uses synchronized official theme icons with transparent antialiased edges and a consistent responsive section rhythm.

Release note: 官网与应用图标现统一使用官方 GoodBuddy 标识，并优化下载区与 Agent Runtime 区之间的页面间距。

sites/assets/favicon.svg

@@ -1,12 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
-  <defs>
-    <linearGradient id="g" x1="8" y1="8" x2="56" y2="56" gradientUnits="userSpaceOnUse">
-      <stop stop-color="#0877e8"/>
-      <stop offset="1" stop-color="#08b89b"/>
-    </linearGradient>
-  </defs>
-  <rect width="64" height="64" rx="16" fill="#fff"/>
-  <path d="M9 34a14 14 0 1 1 28 0v12H23A14 14 0 0 1 9 34Z" fill="none" stroke="url(#g)" stroke-width="7" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M27 34a14 14 0 1 1 28 0 14 14 0 0 1-28 0Z" fill="none" stroke="url(#g)" stroke-width="7"/>
-  <path d="M32 20v-7M41 17l5-5M23 17l-5-5" fill="none" stroke="url(#g)" stroke-width="4" stroke-linecap="round"/>
-</svg>