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/logoMiniDark.svg

@@ -0,0 +1,35 @@
+<svg width="48" height="48" viewBox="0 0 48 48" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="EasyFlow logo mark dark">
+  <defs>
+    <linearGradient id="easyflow-mark-night-top" x1="8%" y1="10%" x2="95%" y2="95%">
+      <stop offset="0%" stop-color="#F5F7FA" />
+      <stop offset="100%" stop-color="#D2D8E1" />
+    </linearGradient>
+    <linearGradient id="easyflow-mark-night-bottom" x1="8%" y1="10%" x2="95%" y2="95%">
+      <stop offset="0%" stop-color="#7A8593" />
+      <stop offset="100%" stop-color="#D5DAE2" />
+    </linearGradient>
+  </defs>
+
+  <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-night-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-night-bottom)"
+    stroke-width="5.8"
+    stroke-linecap="round"
+    stroke-linejoin="round"
+  />
+</svg>