feat: add workday utility functions

- Introduced utility functions for workday calculations in `workday-utils.ts`, including `getPreviousWorkday`, `getNextWorkday`, `isWorkday`, and `getDayName`.
- Updated `DailyService` and `DailyPageClient` to utilize `getPreviousWorkday` for accurate date handling instead of simple date subtraction.

src/app/daily/DailyPageClient.tsx

@@ -10,6 +10,7 @@ import { DailyCalendar } from '@/components/daily/DailyCalendar';
 import { DailySection } from '@/components/daily/DailySection';
 import { dailyClient } from '@/clients/daily-client';
 import { Header } from '@/components/ui/Header';
+import { getPreviousWorkday } from '@/lib/workday-utils';
 
 interface DailyPageClientProps {
   initialDailyView?: DailyView;
@@ -99,9 +100,7 @@ export function DailyPageClient({
   };
 
   const getYesterdayDate = () => {
-    const yesterday = new Date(currentDate);
-    yesterday.setDate(yesterday.getDate() - 1);
-    return yesterday;
+    return getPreviousWorkday(currentDate);
   };
 
   const getTodayDate = () => {