feat: 发薪日期多选+提前N天提醒+电子签设置区域修复

- Schema: 去掉 payrollFrequency，新增 payrollDays (JSON数组) + payrollReminderDays (Int)
- 设置页: 发薪日期改为1-28号多选按钮，新增提前提醒天数设置
- 设置页: 恢复电子签署设置区域（3个开关：规章制度/工资条/入职文件）
- 工作日历: 发薪日期作为 PAYROLL_DAY 事件显示
- 工作台: 提前N天提醒发薪日期，N可配置
- TaskCenter: 新增发薪提醒分类图标
- seed文件: 更新为 payrollDays 格式

backend/prisma/seed-multi-org.ts

@@ -49,7 +49,7 @@ interface OrgConfig {
   adminName: string
   contactName: string
   contactPhone: string
-  payrollFrequency: number
+  payrollDays: number[]
   retirementReminderEnabled: boolean
   socialConfig: {
     city: string
@@ -89,7 +89,7 @@ const ORGS: OrgConfig[] = [
     adminName: '王建国',
     contactName: '王建国',
     contactPhone: '13800000010',
-    payrollFrequency: 1,
+    payrollDays: [10],
     retirementReminderEnabled: true,
     socialConfig: {
       city: '北京',
@@ -122,7 +122,7 @@ const ORGS: OrgConfig[] = [
     adminName: '李明华',
     contactName: '李明华',
     contactPhone: '13800000020',
-    payrollFrequency: 2,
+    payrollDays: [5, 20],
     retirementReminderEnabled: true,
     socialConfig: {
       city: '深圳',
@@ -157,7 +157,7 @@ const ORGS: OrgConfig[] = [
     adminName: '赵雪梅',
     contactName: '赵雪梅',
     contactPhone: '13800000030',
-    payrollFrequency: 1,
+    payrollDays: [10],
     retirementReminderEnabled: false,
     socialConfig: {
       city: '杭州',
@@ -213,7 +213,7 @@ async function createOrg(orgConfig: OrgConfig) {
       city: orgConfig.city,
       contactName: orgConfig.contactName,
       contactPhone: orgConfig.contactPhone,
-      payrollFrequency: orgConfig.payrollFrequency,
+      payrollDays: orgConfig.payrollDays,
       retirementReminderEnabled: orgConfig.retirementReminderEnabled,
     },
   })