feat: 换用官方 logo 原图

- S 标从 2048 原图裁切：导航/页脚/admin 用 512 webp（96KB），favicon 32/64 圆角 png，apple-touch-icon 180
- OG 分享图用完整版 logo-full.jpg（含字标），index.html 补 og:title/description/image
- 自绘 SVG logo 退役；原图 8.4MB 移入 design-assets/（已 gitignore，不进构建产物）
- web/dist 总体积 1.0MB

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

admin/public/favicon.svg

@@ -1,27 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
-  <defs>
-    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
-      <stop offset="0" stop-color="#34E5E0"/>
-      <stop offset="0.5" stop-color="#3B82F6"/>
-      <stop offset="1" stop-color="#A855F7"/>
-    </linearGradient>
-  </defs>
-  <rect width="64" height="64" rx="14" fill="#0B1117"/>
-  <g fill="none" stroke-linecap="round">
-    <path d="M22 15 C41 15 41 31 32 32 C23 33 23 49 42 49" stroke="url(#g)" stroke-width="9" opacity="0.22"/>
-    <path d="M22 15 C41 15 41 31 32 32 C23 33 23 49 42 49" stroke="url(#g)" stroke-width="6" opacity="0.45"/>
-    <path d="M22 15 C41 15 41 31 32 32 C23 33 23 49 42 49" stroke="url(#g)" stroke-width="3.6"/>
-  </g>
-  <g fill="#34E5E0">
-    <circle cx="15" cy="13" r="1.4"/>
-    <circle cx="10" cy="17" r="1"/>
-    <circle cx="14" cy="20" r="0.8"/>
-  </g>
-  <g stroke="#A855F7" stroke-width="0.9" opacity="0.75">
-    <path d="M42 49 L50 46 M42 49 L53 51 M50 46 L53 51" fill="none"/>
-  </g>
-  <g fill="#C084FC">
-    <circle cx="50" cy="46" r="1.5"/>
-    <circle cx="53" cy="51" r="1.2"/>
-  </g>
-</svg>