feat(ui): 新增运营后台站点图标并统一登录页品牌标记

- 新增橙色渐变仪表盘图标（SVG / ICO / apple-touch-icon / 192 / 512）
- 登录页侧栏与移动端品牌标记改用同一 BrandMark
- index.html 补齐图标、theme-color 与页面描述，标题改为「运营后台管理中心」

apps/frontend/src/pages/admin/AdminLoginPage.tsx

@@ -46,6 +46,42 @@ const brandFeatures = [
   },
 ]
 
+/** 与站点图标同源的品牌标记：橙色渐变圆角方块 + 白色仪表盘四宫格。 */
+function BrandMark({ size = 42 }: { size?: number }) {
+  return (
+    <svg
+      width={size}
+      height={size}
+      viewBox="0 0 64 64"
+      role="img"
+      aria-label="运营后台管理中心"
+      style={{ display: 'block', borderRadius: 10 }}
+    >
+      <defs>
+        <linearGradient
+          id="orderBrand"
+          x1="6"
+          y1="2"
+          x2="58"
+          y2="62"
+          gradientUnits="userSpaceOnUse"
+        >
+          <stop offset="0" stopColor="#FB923C" />
+          <stop offset="0.5" stopColor="#F97316" />
+          <stop offset="1" stopColor="#EA580C" />
+        </linearGradient>
+      </defs>
+      <rect width="64" height="64" rx="17" fill="url(#orderBrand)" />
+      <g fill="#fff">
+        <rect x="15" y="15" width="15" height="19" rx="4.5" />
+        <rect x="34" y="15" width="15" height="13" rx="4.5" />
+        <rect x="15" y="38" width="15" height="11" rx="4.5" opacity="0.72" />
+        <rect x="34" y="32" width="15" height="17" rx="4.5" opacity="0.9" />
+      </g>
+    </svg>
+  )
+}
+
 export default function AdminLoginPage() {
   const navigate = useNavigate()
   const { message } = App.useApp()
@@ -188,7 +224,7 @@ export default function AdminLoginPage() {
         <aside className="auth-brand">
           <div className="auth-brand-logo">
             <span className="auth-logo-mark">
-              <DashboardOutlined />
+              <BrandMark size={42} />
             </span>
             <strong>Order Site Admin</strong>
           </div>
@@ -218,7 +254,7 @@ export default function AdminLoginPage() {
           <div className="auth-form-card">
             <div className="auth-mobile-brand">
               <span className="auth-logo-mark">
-                <DashboardOutlined />
+                <BrandMark size={36} />
               </span>
               <strong>Order Site Admin</strong>
             </div>