style: 移动端流转树主干道对齐与连线箭头

- 主线(含 TRANSFER 子任务)左边缘对齐,消除缩进形成笔直主干道
- 主线卡片之间加向下连线箭头(↓)
- 分支(SPAWN 协助)保留缩进 + 左边线

track-uniapp/src/pages/scan/components/TreeCanvas.vue

@@ -16,16 +16,18 @@
 
     <scroll-view v-else scroll-y class="tree-scroll" :style="{ height: scrollHeight + 'px' }">
       <view class="tree-root">
-        <!-- 递归纵向流转树：主线 + 分支缩进 -->
-        <FlowTree
-          v-for="t in product.task_tree"
-          :key="t.id"
-          :node="t"
-          :current-user="currentUser"
-          :current-user-id="currentUserId"
-          :current-username="currentUsername"
-          @view-records="$emit('viewRecords', $event)"
-        />
+        <!-- 递归纵向流转树：根级主线之间用 ↓ 连线，分支由 FlowTree 内部缩进 -->
+        <template v-for="(t, i) in product.task_tree">
+          <view :key="'arrow-' + i" v-if="i > 0" class="ft-flow-arrow">↓</view>
+          <FlowTree
+            :key="t.id"
+            :node="t"
+            :current-user="currentUser"
+            :current-user-id="currentUserId"
+            :current-username="currentUsername"
+            @view-records="$emit('viewRecords', $event)"
+          />
+        </template>
       </view>
 
       <!-- 图例 -->
@@ -96,6 +98,15 @@ export default {
 
 .tree-scroll { flex-shrink: 0; }
 .tree-root { padding: 16px 12px 20px; }
+/* 根级主线之间的向下连线箭头 */
+.ft-flow-arrow {
+  text-align: center;
+  color: #9ca3af;
+  font-size: 16px;
+  font-weight: 700;
+  line-height: 1;
+  margin: 2px 0;
+}
 .tree-legend {
   display: flex;
   flex-wrap: wrap;