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/logoMono.svg

@@ -0,0 +1,38 @@
+<svg width="180" height="48" viewBox="0 0 180 48" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="EasyFlow logo mono">
+  <g transform="translate(0,2)">
+    <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="#111827"
+      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="#111827"
+      stroke-width="5.8"
+      stroke-linecap="round"
+      stroke-linejoin="round"
+    />
+  </g>
+
+  <text
+    x="56"
+    y="31"
+    fill="#111827"
+    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>