R47 A1-A8：完成薪资管理模块

client/src/api/employees.ts

@@ -250,6 +250,20 @@ export type EmployeeStatsResponse = {
     validHolderCount: number;
     expiringSoonCount: number;
   };
+  salarySummary: {
+    currentMonth: {
+      month: string;
+      totalNetAmount: number;
+      averageNetAmount: number;
+      issuedCount: number;
+    } | null;
+    previousMonth: {
+      month: string;
+      totalNetAmount: number;
+      averageNetAmount: number;
+      issuedCount: number;
+    } | null;
+  } | null;
   performanceSummary: {
     cycle: {
       id: string;