fix: 出题页新建会覆盖最后一道题；自由创作预览数据存到了错误的任务下

- 教程 / 挑战编辑器“新建”时序号没有加 1（?? 的优先级），而保存按序号更新，
  新建后保存会覆盖列表里最后一个任务
- 预览的 localStorage 作用域改由页面传入任务 id；自由创作页用的是局部 taskId，
  之前读全局 taskId 会把数据存到上一个打开过的任务名下

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/src/components/editor/Preview.vue

@@ -67,7 +67,6 @@ import {
   listenPreviewStorage,
   loadPreviewStorage,
 } from "../../utils/previewStorage"
-import { taskId } from "../../store/task"
 import { user } from "../../store/user"
 
 interface Props {
@@ -78,8 +77,8 @@ interface Props {
   submissionId?: string
   showCodeButton?: boolean
   clearable?: boolean
-  // 自己的编辑器：代码里的 localStorage 按“用户 + 任务”存下来，刷新后还在
-  persistStorage?: boolean
+  // 自己的编辑器传当前任务 id：代码里的 localStorage 按“用户 + 任务”存下来，刷新后还在
+  storageTaskId?: number
 }
 
 const props = defineProps<Props>()
@@ -129,7 +128,7 @@ const previewContent = ref("")
 
 const frame = useTemplateRef<HTMLIFrameElement>("frame")
 const storageScope = () =>
-  props.persistStorage && taskId.value ? `${user.username}:${taskId.value}` : ""
+  props.storageTaskId ? `${user.username}:${props.storageTaskId}` : ""
 
 function getContent(forDownload = false) {
   const scope = storageScope()

client/src/pages/ChallengeDetail.vue

@@ -90,7 +90,7 @@
         :css="css"
         :js="js"
         :asset-base-url="assetBaseUrl"
-        persist-storage
+        :storage-task-id="taskId"
         show-code-button
         clearable
         @showCode="showCode = true"

client/src/pages/ChallengeEditor.vue

@@ -224,7 +224,7 @@ async function getContent() {
 }
 
 function createNew() {
-  challenge.display = list.value[list.value.length - 1]?.display ?? 0 + 1
+  challenge.display = (list.value[list.value.length - 1]?.display ?? 0) + 1
   challenge.title = ""
   challenge.content = ""
   challenge.score = 0

client/src/pages/CreationDetail.vue

@@ -80,7 +80,7 @@
         :html="html"
         :css="css"
         :js="js"
-        persist-storage
+        :storage-task-id="taskId"
         show-code-button
         clearable
         @showCode="showCode = true"

client/src/pages/TutorialEditor.vue

@@ -192,7 +192,7 @@ async function getContent() {
 }
 
 function createNew() {
-  tutorial.display = list.value[list.value.length - 1]?.display ?? 0 + 1
+  tutorial.display = (list.value[list.value.length - 1]?.display ?? 0) + 1
   tutorial.title = ""
   tutorial.content = ""
   tutorial.is_public = false

client/src/pages/Workspace.vue

@@ -14,7 +14,7 @@
             :css="css"
             :js="js"
             :asset-base-url="assetBaseUrl"
-            persist-storage
+            :storage-task-id="taskId"
           />
         </template>
       </n-split>
@@ -28,7 +28,7 @@ import Preview from "../components/editor/Preview.vue"
 import TaskPanel from "../components/task/TaskPanel.vue"
 import { show, panelSize } from "../store/panel"
 import { html, css, js } from "../store/editors"
-import { assetBaseUrl } from "../store/task"
+import { assetBaseUrl, taskId } from "../store/task"
 
 const { ctrl_s } = useMagicKeys({
   passive: false,