feat: 禁用默认代理复选框以防止用户修改

app/pages/admin/agents/[slug].vue

@@ -283,7 +283,7 @@ onMounted(async () => {
             </div>
           </label>
           <label class="toggle-item">
-            <input type="checkbox" :checked="form.isDefault === 1" @change="form.isDefault = form.isDefault ? 0 : 1" />
+            <input type="checkbox" disabled :checked="form.isDefault === 1" @change="form.isDefault = form.isDefault ? 0 : 1" />
             <div class="toggle-info">
               <div class="toggle-name">默认 Agent</div>
               <div class="toggle-desc">设为新用户/未指定时的默认 Agent</div>