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/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;
+}
+