fix: 允许设备购买日期和保修到期日期为空

修改后端模型和前端表单字段，将购买日期和保修到期日期设为非必填项，以适应实际业务中可能缺少这些信息的情况

backend/models/Device.js

@@ -53,11 +53,11 @@ const Device = sequelize.define('Device', {
   },
   purchaseDate: {
     type: DataTypes.DATE,
-    allowNull: false
+    allowNull: true
   },
   warrantyExpiry: {
     type: DataTypes.DATE,
-    allowNull: false
+    allowNull: true
   },
   ipAddress: {
     type: DataTypes.STRING

frontend/src/pages/DeviceManagement.jsx

@@ -179,8 +179,8 @@ const defaultDeviceFields = [
   { fieldName: 'powerConsumption', displayName: '功率(W)', fieldType: 'number', required: true, order: 9, visible: true },
   { fieldName: 'status', displayName: '状态', fieldType: 'select', required: true, order: 10, visible: true, 
     options: [{ value: 'running', label: '运行中' }, { value: 'maintenance', label: '维护中' }, { value: 'offline', label: '离线' }, { value: 'fault', label: '故障' }] },
-  { fieldName: 'purchaseDate', displayName: '购买日期', fieldType: 'date', required: true, order: 11, visible: true },
-  { fieldName: 'warrantyExpiry', displayName: '保修到期', fieldType: 'date', required: true, order: 12, visible: true },
+  { fieldName: 'purchaseDate', displayName: '购买日期', fieldType: 'date', required: false, order: 11, visible: true },
+  { fieldName: 'warrantyExpiry', displayName: '保修到期', fieldType: 'date', required: false, order: 12, visible: true },
   { fieldName: 'ipAddress', displayName: 'IP地址', fieldType: 'string', required: false, order: 13, visible: true },
   { fieldName: 'description', displayName: '描述', fieldType: 'textarea', required: false, order: 14, visible: true }
 ];