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/logoMini.svg

@@ -1,16 +1,35 @@
-<svg width="24" height="22" viewBox="0 0 24 22" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="EasyFlow logo mini">
+<svg width="48" height="48" viewBox="0 0 48 48" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="EasyFlow logo mark">
   <defs>
-    <linearGradient id="efMiniGrad" x1="0" y1="0" x2="1" y2="1">
-      <stop offset="0%" stop-color="#2563EB"/>
-      <stop offset="100%" stop-color="#06B6D4"/>
+    <linearGradient id="easyflow-mark-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-mark-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="0.5" y="0.5" width="21" height="21" rx="6" fill="url(#efMiniGrad)"/>
-  <path d="M6 7.5h5.5m5 0H17m-7.5 7h5.5m-5.5 0H6" stroke="#FFFFFF" stroke-width="1.8" stroke-linecap="round"/>
-  <path d="M11.5 7.5v7m0 0l4.2-3.2" stroke="#FFFFFF" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"/>
-  <circle cx="6" cy="7.5" r="1.8" fill="#FFFFFF"/>
-  <circle cx="17" cy="7.5" r="1.8" fill="#FFFFFF"/>
-  <circle cx="11.5" cy="14.5" r="1.8" fill="#FFFFFF"/>
-  <circle cx="6" cy="14.5" r="1.8" fill="#FFFFFF"/>
+  <path
+    d="M 11 10.8
+       C 11 24, 21.8 26.4, 29 26.4
+       C 33.8 26.4, 36.2 21.6, 32.6 18
+       C 29 14.4, 21.8 16.8, 17 21.6"
+    fill="none"
+    stroke="url(#easyflow-mark-top)"
+    stroke-width="5.8"
+    stroke-linecap="round"
+    stroke-linejoin="round"
+  />
+  <path
+    d="M 37 37.2
+       C 37 24, 26.2 21.6, 19 21.6
+       C 14.2 21.6, 11.8 26.4, 15.4 30
+       C 19 33.6, 26.2 31.2, 31 26.4"
+    fill="none"
+    stroke="url(#easyflow-mark-bottom)"
+    stroke-width="5.8"
+    stroke-linecap="round"
+    stroke-linejoin="round"
+  />
 </svg>