品牌真正接入全站渲染 + document.title 同步 + 设置页实时刷新

- BrandContext 全局上下文: 挂载时拉取 GET /api/settings/brand
- HomePage: 品牌名称/标语/Logo 动态渲染, 封面卡片精调
- AdminLayout: 侧栏品牌名称/顶栏标语动态显示
- LoginPage: 登录页品牌 Logo + 名称
- WatchPage: 播放页接入品牌上下文
- Settings: 保存后调用 refresh() 全站即时刷新
- App.tsx: document.title 跟随 brand_name 变化
- 组件分层: AppRoutes + BrandProvider 避免 import 循环

frontend/src/admin/AdminLayout.tsx

@@ -6,6 +6,7 @@ import {
   CloudServerOutlined, SettingOutlined, LogoutOutlined,
 } from '@ant-design/icons'
 import { clearToken } from '../api/client'
+import { useBrand } from '../brand/BrandContext'
 
 const { Sider, Content, Header } = Layout
 const { Text } = Typography
@@ -19,6 +20,7 @@ const menuItems = [
 ]
 
 export default function AdminLayout() {
+  const { brand } = useBrand()
   const [collapsed, setCollapsed] = useState(false)
   const navigate = useNavigate()
   const location = useLocation()
@@ -48,7 +50,7 @@ export default function AdminLayout() {
             fontFamily: '"Source Serif 4", "Noto Serif SC", Georgia, serif',
             color: '#C2693A',
           }}>
-            {collapsed ? 'SC' : 'StreamCore'}
+            {collapsed ? brand.brand_name.slice(0, 2) : brand.brand_name}
           </Text>
         </div>
         <Menu
@@ -60,9 +62,10 @@ export default function AdminLayout() {
       <Layout>
         <Header style={{
           background: '#FBF9F6', borderBottom: '1px solid #E8E3D8',
-          display: 'flex', justifyContent: 'flex-end', alignItems: 'center',
+          display: 'flex', justifyContent: 'space-between', alignItems: 'center',
           padding: '0 40px',
         }}>
+          <Text type="secondary" style={{ fontSize: 13 }}>{brand.brand_tagline}</Text>
           <Button type="text" icon={<LogoutOutlined />} onClick={handleLogout}>
             退出登录
           </Button>