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/scripts/validate.mjs

@@ -9,7 +9,8 @@ const requiredFiles = [
   "index.html",
   "styles.css",
   "app.js",
-  "assets/favicon.svg",
+  "assets/goodbuddy-light.png",
+  "assets/goodbuddy-dark.png",
   "README.md",
 ];
 
@@ -67,6 +68,15 @@ report(/class="skip-link"\s+href="#main-content"/.test(html), "缺少跳到主
 report(/<main\s+id="main-content">/.test(html), "缺少 main-content 主区域");
 report(/aria-label="主导航"/.test(html), "主导航缺少可访问名称");
 report(/data-theme-toggle/.test(html), "缺少主题切换控件");
+report(
+  (html.match(/src="\.\/assets\/goodbuddy-light\.png"/g) ?? []).length >= 5,
+  "品牌位置必须使用官方亮色图标",
+);
+report(
+  (html.match(/src="\.\/assets\/goodbuddy-dark\.png"/g) ?? []).length >= 5,
+  "品牌位置必须使用官方深色图标",
+);
+report(!/class="brand-mark"/.test(html), "官网不得使用自绘品牌标志");
 report(/data-tilt-stage/.test(html), "首屏产品界面缺少倾斜交互区域");
 report(/data-tilt-card/.test(html), "首屏产品界面缺少倾斜卡片");
 report(/prefers-reduced-motion:\s*reduce/.test(css), "缺少减少动态效果规则");