fix(前端): 修复成员表头与加班导出

关键改动：

- 成员管理表格改为卡片内滚动，并固定每个表头单元格

- 加班导出按当前月份筛选导出记录

- 增加成员表头与加班导出月份过滤测试

Co-Authored-By: Codex GPT-5 <codex@openai.com>

apps/web/lib/overtime.ts

@@ -118,6 +118,14 @@ export function calcDuration(start: string, end: string): number {
   return Math.round(total * 10) / 10;
 }
 
+export function filterOvertimeRecordsForExportMonth(
+  records: OvertimeRecord[],
+  monthFilter: string,
+): OvertimeRecord[] {
+  if (!monthFilter) return [...records];
+  return records.filter((record) => record.startTime.slice(0, 7) === monthFilter);
+}
+
 function collectDepartmentTreeIds(departments: Department[], departmentId: string): Set<string> {
   const ids = new Set<string>([departmentId]);
   let changed = true;