feat(linkage): 鞍座改为预备生产位 + 在线改人工触发 + 计划删除

- 上卷鞍座=预备生产位（不生产）；投入生产后离开鞍座、进入生产中并转入物料跟踪，鞍座随即空出
- 在线状态改为人工触发：移动到入口端才变在线，引擎不再自动上线(ensure_online 置空)
- 单卷在产：投入生产时若已有在产卷则拒绝
- 物料跟踪显示在产卷实时进度(客户端外推带头长度/进度条)
- 入口跟踪鞍座卡片改为预备生产展示(去掉进度)
- 计划管理新增删除按钮 + DELETE /plan/{id}(生产中不可删)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/api/index.js

@@ -19,6 +19,7 @@ export const updateProductionRecord = (id, data) => request.put(`/production/${i
 export const getPlans = params => request.get('/plan/', { params })
 export const createPlan = data => request.post('/plan/', data)
 export const updatePlan = (id, data) => request.put(`/plan/${id}`, data)
+export const deletePlan = id => request.delete(`/plan/${id}`)
 export const confirmPlan = id => request.patch(`/plan/${id}/confirm`)
 export const startProducing = id => request.patch(`/plan/${id}/start`)   // 移动到上卷鞍座(兼容)
 export const moveToSaddle = id => request.patch(`/plan/${id}/start`)