Extract weekend detection logic to shared utility

- Created isWeekend() function in utils/date.ts
- Replaced inline weekend detection logic in availability page
- More reusable and testable

frontend/app/admin/availability/page.tsx

@@ -8,7 +8,7 @@ import { Header } from "../../components/Header";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
 import { components } from "../../generated/api";
 import constants from "../../../../shared/constants.json";
-import { formatDate, formatDisplayDate, getDateRange, formatTimeString } from "../../utils/date";
+import { formatDate, formatDisplayDate, getDateRange, formatTimeString, isWeekend } from "../../utils/date";
 
 const { slotDurationMinutes, maxAdvanceDays, minAdvanceDays } = constants.booking;
 
@@ -260,7 +260,7 @@ export default function AdminAvailabilityPage() {
               const hasSlots = slots.length > 0;
               const isSource = copySource === dateStr;
               const isTarget = copyTargets.has(dateStr);
-              const isWeekend = date.getDay() === 0 || date.getDay() === 6;
+              const isWeekendDate = isWeekend(date);
 
               return (
                 <div
@@ -271,7 +271,7 @@ export default function AdminAvailabilityPage() {
                     ...(hasSlots ? styles.dayCardActive : {}),
                     ...(isSource ? styles.dayCardSource : {}),
                     ...(isTarget ? styles.dayCardTarget : {}),
-                    ...(isWeekend ? styles.dayCardWeekend : {}),
+                    ...(isWeekendDate ? styles.dayCardWeekend : {}),
                   }}
                   onClick={() => {
                     if (copySource) {

frontend/app/utils/date.ts

@@ -74,3 +74,11 @@ export function getDateRange(minAdvanceDays: number, maxAdvanceDays: number): Da
   return dates;
 }
 
+/**
+ * Check if a date is a weekend (Saturday or Sunday).
+ */
+export function isWeekend(date: Date): boolean {
+  const day = date.getDay();
+  return day === 0 || day === 6;
+}
+