改派页增加/删除槽位:addAssignmentSlot 封装 + AssignmentSlotAddModal 弹窗 + 删除走既有 deleteAssignmentSlot 跟随后端 canDelete;改派槽位表切到 resolveBoardAssignmentSlots 覆盖未派占位行;增删后刷新提示需重新确认。