fix: 自助修改密码后断开 VPN 连接并登出所有 Web 会话

- auth.go: ChangePassword 失效所有会话（不再排除当前会话），调用 KickUser 断开 VPN 隧道
- ProfileView.vue: 改密成功后 logout 清除 token，跳转登录页并携带 msg 参数
- LoginView.vue: 读取 query.msg 显示"密码已修改，请重新登录"绿色提示
- zh.ts/en.ts: 新增 login.passwordChanged 国际化 key

frontend/src/views/LoginView.vue

@@ -1,16 +1,18 @@
 <script setup lang="ts">
 import { ref } from 'vue'
-import { useRouter } from 'vue-router'
+import { useRouter, useRoute } from 'vue-router'
 import { useI18n } from 'vue-i18n'
 import { useAuthStore } from '@/stores/auth'
 
 const router = useRouter()
+const route = useRoute()
 const authStore = useAuthStore()
 const { t } = useI18n()
 
 const username = ref('')
 const password = ref('')
 const error = ref('')
+const successMsg = ref(route.query.msg === 'password_changed' ? t('login.passwordChanged') : '')
 const loading = ref(false)
 
 async function handleLogin() {
@@ -71,6 +73,7 @@ async function handleLogin() {
           />
         </div>
         <p v-if="error" class="text-sm text-red-500">{{ error }}</p>
+        <p v-if="successMsg" class="text-sm text-green-500">{{ successMsg }}</p>
         <button
           type="submit"
           :disabled="loading"