feat: 实现登录功能，包括用户名和密码输入、表单验证及错误处理

frontend/portal/src/service/auth.js

@@ -1,6 +1,16 @@
 import { requestJson } from './apiClient';
 import { setTokenAndLoadMe } from './session';
 
+export async function login({ username, password }) {
+    const data = await requestJson('/v1/auth/login', {
+        method: 'POST',
+        body: { username, password }
+    });
+    const token = data?.token ?? '';
+    if (token) await setTokenAndLoadMe(token);
+    return token;
+}
+
 export async function register({ username, password, confirmPassword, verifyCode }) {
     const data = await requestJson('/v1/auth/register', {
         method: 'POST',