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/logo.svg

@@ -1,20 +1,49 @@
-<svg width="120" height="27" viewBox="0 0 120 27" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="EasyFlow logo">
+<svg width="180" height="48" viewBox="0 0 180 48" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="EasyFlow logo">
   <defs>
-    <linearGradient id="efGrad" x1="0" y1="0" x2="1" y2="1">
-      <stop offset="0%" stop-color="#2563EB"/>
-      <stop offset="100%" stop-color="#06B6D4"/>
+    <linearGradient id="easyflow-water-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-water-bottom" x1="10%" y1="10%" x2="95%" y2="95%">
+      <stop offset="0%" stop-color="#3A7BD5" />
+      <stop offset="100%" stop-color="#00D2FF" />
     </linearGradient>
   </defs>
 
   <g transform="translate(0,2)">
-    <rect x="0" y="0" width="23" height="23" rx="6" fill="url(#efGrad)"/>
-    <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-water-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-water-bottom)"
+      stroke-width="5.8"
+      stroke-linecap="round"
+      stroke-linejoin="round"
+    />
   </g>
 
-  <text x="30" y="18" fill="#0F172A" font-family="Segoe UI, Arial, sans-serif" font-size="13" font-weight="700" letter-spacing="0.2">EasyFlow</text>
+  <text
+    x="56"
+    y="31"
+    fill="#0F172A"
+    font-family="'SF Pro Display', 'Segoe UI', 'PingFang SC', 'Hiragino Sans GB', Arial, sans-serif"
+    font-size="21"
+    font-weight="700"
+    letter-spacing="0.2"
+  >
+    EasyFlow
+  </text>
 </svg>