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/viewer/pages/WatchPage.tsx

@@ -4,10 +4,12 @@ import { Select, Button, Typography, Space, Tag, Spin, Skeleton, Empty, message
 import { ArrowLeftOutlined, WifiOutlined, SignalFilled } from '@ant-design/icons'
 import Hls from 'hls.js'
 import { getWatchUrls, type WatchInfo, type LineInfo } from '../../api/streams'
+import { useBrand } from '../../brand/BrandContext'
 
 const { Title, Text } = Typography
 
 export default function WatchPage() {
+  const { brand } = useBrand()
   const navigate = useNavigate()
   const { id } = useParams<{ id: string }>()
   const videoRef = useRef<HTMLVideoElement>(null)