feat: 完善代理商端业务逻辑与前后端框架

主要更新：
- 更新代理商端文档，明确项目由品牌方分配流程
- 新增Brief配置详情页(已配置)设计稿
- 完善工作台紧急待办中品牌新任务功能
- 整理Pencil设计文件中代理商端页面顺序
- 新增后端FastAPI框架及核心API
- 新增前端Next.js页面和组件库
- 添加.gitignore排除构建和缓存文件

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/contexts/AuthContext.tsx

@@ -0,0 +1,118 @@
+'use client'
+
+import { createContext, useContext, useState, useEffect, ReactNode } from 'react'
+import { User, UserRole, LoginCredentials, AuthContextType } from '@/types/auth'
+
+const AuthContext = createContext<AuthContextType | undefined>(undefined)
+
+// 模拟用户数据（后续替换为真实 API）
+const MOCK_USERS: Record<string, User & { password: string }> = {
+  'creator@demo.com': {
+    id: 'user-001',
+    name: '达人小美',
+    email: 'creator@demo.com',
+    role: 'creator',
+    tenantId: 'tenant-001',
+    tenantName: '美妆品牌A',
+    password: 'demo123',
+  },
+  'agency@demo.com': {
+    id: 'user-002',
+    name: '张经理',
+    email: 'agency@demo.com',
+    role: 'agency',
+    tenantId: 'tenant-001',
+    tenantName: '美妆品牌A',
+    password: 'demo123',
+  },
+  'brand@demo.com': {
+    id: 'user-003',
+    name: '李总监',
+    email: 'brand@demo.com',
+    role: 'brand',
+    tenantId: 'tenant-001',
+    tenantName: '美妆品牌A',
+    password: 'demo123',
+  },
+}
+
+const STORAGE_KEY = 'miaosi_auth'
+
+export function AuthProvider({ children }: { children: ReactNode }) {
+  const [user, setUser] = useState<User | null>(null)
+  const [isLoading, setIsLoading] = useState(true)
+  const [mounted, setMounted] = useState(false)
+
+  // 初始化时从 localStorage 恢复登录状态
+  useEffect(() => {
+    setMounted(true)
+    if (typeof window !== 'undefined') {
+      const stored = localStorage.getItem(STORAGE_KEY)
+      if (stored) {
+        try {
+          const parsed = JSON.parse(stored)
+          setUser(parsed)
+        } catch {
+          localStorage.removeItem(STORAGE_KEY)
+        }
+      }
+    }
+    setIsLoading(false)
+  }, [])
+
+  const login = async (credentials: LoginCredentials): Promise<{ success: boolean; error?: string }> => {
+    // 模拟 API 延迟
+    await new Promise((resolve) => setTimeout(resolve, 500))
+
+    const mockUser = MOCK_USERS[credentials.email]
+    if (!mockUser) {
+      return { success: false, error: '用户不存在' }
+    }
+    if (mockUser.password !== credentials.password) {
+      return { success: false, error: '密码错误' }
+    }
+
+    // eslint-disable-next-line @typescript-eslint/no-unused-vars
+    const { password, ...userWithoutPassword } = mockUser
+    setUser(userWithoutPassword)
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(userWithoutPassword))
+
+    return { success: true }
+  }
+
+  const logout = () => {
+    setUser(null)
+    localStorage.removeItem(STORAGE_KEY)
+  }
+
+  const switchRole = (role: UserRole) => {
+    if (user) {
+      const updated = { ...user, role }
+      setUser(updated)
+      localStorage.setItem(STORAGE_KEY, JSON.stringify(updated))
+    }
+  }
+
+  return (
+    <AuthContext.Provider
+      value={{
+        user,
+        isAuthenticated: !!user,
+        isLoading,
+        login,
+        logout,
+        switchRole,
+      }}
+    >
+      {children}
+    </AuthContext.Provider>
+  )
+}
+
+export function useAuth() {
+  const context = useContext(AuthContext)
+  if (context === undefined) {
+    throw new Error('useAuth must be used within an AuthProvider')
+  }
+  return context
+}