feat: 更新管理端品牌 Logo 体系

- 替换主标、暗色主标、收起图标、单色版与 favicon 资源

- 新增暗色收起图标接入能力，补齐公共 logo 配置回退链路

- 增加品牌预览页，集中展示浅色、黑场和应用场景效果

easyflow-ui-admin/app/public/favicon.svg

@@ -1,16 +1,44 @@
 <svg width="64" height="64" viewBox="0 0 64 64" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="EasyFlow favicon">
   <defs>
-    <linearGradient id="efFavGrad" x1="0" y1="0" x2="1" y2="1">
-      <stop offset="0%" stop-color="#1D4ED8"/>
-      <stop offset="100%" stop-color="#0891B2"/>
+    <linearGradient id="easyflow-favicon-bg" x1="10%" y1="8%" x2="90%" y2="92%">
+      <stop offset="0%" stop-color="#11151B" />
+      <stop offset="100%" stop-color="#06080B" />
+    </linearGradient>
+    <linearGradient id="easyflow-favicon-top" x1="10%" y1="10%" x2="95%" y2="95%">
+      <stop offset="0%" stop-color="#00C9FF" />
+      <stop offset="100%" stop-color="#92F3D0" />
+    </linearGradient>
+    <linearGradient id="easyflow-favicon-bottom" x1="10%" y1="10%" x2="95%" y2="95%">
+      <stop offset="0%" stop-color="#3A7BD5" />
+      <stop offset="100%" stop-color="#00D2FF" />
     </linearGradient>
   </defs>
 
-  <rect x="4" y="4" width="56" height="56" rx="16" fill="url(#efFavGrad)"/>
-  <path d="M17 22h15m14 0h1m-16 20h15m-15 0H17" stroke="#FFFFFF" stroke-width="4" stroke-linecap="round"/>
-  <path d="M32 22v20m0 0l11-8" stroke="#FFFFFF" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
-  <circle cx="17" cy="22" r="4" fill="#FFFFFF"/>
-  <circle cx="47" cy="22" r="4" fill="#FFFFFF"/>
-  <circle cx="32" cy="42" r="4" fill="#FFFFFF"/>
-  <circle cx="17" cy="42" r="4" fill="#FFFFFF"/>
+  <rect x="4" y="4" width="56" height="56" rx="18" fill="url(#easyflow-favicon-bg)" />
+  <rect x="4.5" y="4.5" width="55" height="55" rx="17.5" fill="none" stroke="#FFFFFF" stroke-opacity="0.08" />
+
+  <g transform="translate(0,1)">
+    <path
+      d="M 16 15
+         C 16 32, 29.8 35.2, 39 35.2
+         C 45.1 35.2, 48.2 29.1, 43.6 24.5
+         C 39 19.9, 29.8 23, 23.7 29.1"
+      fill="none"
+      stroke="url(#easyflow-favicon-top)"
+      stroke-width="7"
+      stroke-linecap="round"
+      stroke-linejoin="round"
+    />
+    <path
+      d="M 48 49
+         C 48 32, 34.2 28.9, 25 28.9
+         C 18.9 28.9, 15.8 35, 20.4 39.6
+         C 25 44.2, 34.2 41.1, 40.3 35"
+      fill="none"
+      stroke="url(#easyflow-favicon-bottom)"
+      stroke-width="7"
+      stroke-linecap="round"
+      stroke-linejoin="round"
+    />
+  </g>
 </svg>