perf(frontend): Context value useMemo 稳定化

AuthContext.Provider 和 ToastContext.Provider 的 value 对象
用 useMemo 包裹，避免每次 Provider render 产生新引用导致
所有消费者不必要的 re-render。

frontend/src/components/ui/Toast.tsx

@@ -3,6 +3,7 @@ import {
   useContext,
   useState,
   useCallback,
+  useMemo,
   type ReactNode,
 } from "react";
 import { X, CheckCircle, AlertCircle, Info } from "lucide-react";
@@ -52,8 +53,11 @@ export function ToastProvider({ children }: { children: ReactNode }) {
     }, 3500);
   }, []);
 
+  // 🚀 稳定 Context value 引用
+  const ctxValue = useMemo<ToastContextValue>(() => ({ toast }), [toast]);
+
   return (
-    <ToastContext.Provider value={{ toast }}>
+    <ToastContext.Provider value={ctxValue}>
       {children}
 
       {/* Toast 渲染区 */}

frontend/src/contexts/AuthContext.tsx

@@ -3,6 +3,7 @@ import {
   useContext,
   useState,
   useCallback,
+  useMemo,
   useEffect,
   type ReactNode,
 } from "react";
@@ -107,15 +108,19 @@ export function AuthProvider({ children }: { children: ReactNode }) {
     logoutInternal();
   }, []);
 
+  // 🚀 稳定 Context value 引用，避免消费者不必要的 re-render
+  const ctxValue = useMemo<AuthContextValue>(
+    () => ({
+      ...state,
+      login,
+      logout,
+      isAuthenticated: !!state.token && !!state.user,
+    }),
+    [state, login, logout],
+  );
+
   return (
-    <AuthContext.Provider
-      value={{
-        ...state,
-        login,
-        logout,
-        isAuthenticated: !!state.token && !!state.user,
-      }}
-    >
+    <AuthContext.Provider value={ctxValue}>
       {children}
     </AuthContext.Provider>
   );