fix(frontend): remove unused default React imports and use typed FC/FormEvent where needed

frontend/src/context/AuthContext.tsx

@@ -1,8 +1,8 @@
-import React, { useState, useEffect } from 'react';
+import { useState, useEffect, type ReactNode, type FC } from 'react';
 import client from '../api/client';
 import { AuthContext, User } from './AuthContextValue';
 
-export const AuthProvider: React.FC<{ children: React.ReactNode }> = ({ children }) => {
+export const AuthProvider: FC<{ children: ReactNode }> = ({ children }) => {
   const [user, setUser] = useState<User | null>(null);
   const [isLoading, setIsLoading] = useState(true);