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/brand/BrandContext.tsx

@@ -0,0 +1,55 @@
+import { createContext, useContext, useState, useEffect, useCallback, type ReactNode } from 'react'
+import api from '../api/client'
+
+interface BrandData {
+  brand_name: string
+  brand_tagline: string
+  brand_logo_url: string
+}
+
+interface BrandContextType {
+  brand: BrandData
+  loading: boolean
+  refresh: () => Promise<void>
+}
+
+const defaultBrand: BrandData = {
+  brand_name: 'StreamCore',
+  brand_tagline: '企业级直播平台',
+  brand_logo_url: '',
+}
+
+const BrandContext = createContext<BrandContextType>({
+  brand: defaultBrand,
+  loading: false,
+  refresh: async () => {},
+})
+
+export function BrandProvider({ children }: { children: ReactNode }) {
+  const [brand, setBrand] = useState<BrandData>(defaultBrand)
+  const [loading, setLoading] = useState(true)
+
+  const refresh = useCallback(async () => {
+    try {
+      const { data } = await api.get('/settings/brand')
+      setBrand({
+        brand_name: data.brand_name || defaultBrand.brand_name,
+        brand_tagline: data.brand_tagline || defaultBrand.brand_tagline,
+        brand_logo_url: data.brand_logo_url || '',
+      })
+    } catch { /* use default */ }
+    setLoading(false)
+  }, [])
+
+  useEffect(() => { refresh() }, [refresh])
+
+  return (
+    <BrandContext.Provider value={{ brand, loading, refresh }}>
+      {children}
+    </BrandContext.Provider>
+  )
+}
+
+export function useBrand() {
+  return useContext(BrandContext)
+}