feat: 品牌改版 v2 — 跟随霓虹数据流 logo

- logo 矢量化：青→蓝→紫渐变 S 数据流（logo.tsx + favicon.svg，web/admin 共用）
- 全站配色跟随 logo：强调色青瓷绿 → 青蓝（浅 #0284C7 / 暗 #38BDF8），中性色转冷蓝，终端窗深岩蓝底
- 紫色克制使用：仅 logo 与 .text-brand-gradient（Hero 标题"AI Agent 工作台"渐变字）
- 导航/页脚/admin 顶栏/登录页挂载 logo 标记
- 终端窗与代码块的提示符、光标、事件标签同步换色

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

web/public/favicon.svg

@@ -1,5 +1,27 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
-  <rect width="32" height="32" rx="7" fill="#10201b"/>
-  <path d="M9 20.5 15 10l3.2 5.6L21 11l2.5 4.3" fill="none" stroke="#3ecdad" stroke-width="2.4" stroke-linecap="round" stroke-linejoin="round"/>
-  <circle cx="23.5" cy="15.3" r="1.6" fill="#3ecdad"/>
+<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>