Make error handling consistent across frontend

- Standardized all error handling to use ternary pattern
- Changed if/else blocks to ternary operators for consistency
- Updated booking, appointments, and admin appointments pages

frontend/app/admin/appointments/page.tsx

@@ -202,11 +202,7 @@ export default function AdminAppointmentsPage() {
       await fetchAppointments();
       setConfirmCancelId(null);
     } catch (err) {
-      if (err instanceof Error) {
-        setError(err.message);
-      } else {
-        setError("Failed to cancel appointment");
-      }
+      setError(err instanceof Error ? err.message : "Failed to cancel appointment");
     } finally {
       setCancellingId(null);
     }

frontend/app/appointments/page.tsx

@@ -194,11 +194,7 @@ export default function AppointmentsPage() {
       await fetchAppointments();
       setConfirmCancelId(null);
     } catch (err) {
-      if (err instanceof Error) {
-        setError(err.message);
-      } else {
-        setError("Failed to cancel appointment");
-      }
+      setError(err instanceof Error ? err.message : "Failed to cancel appointment");
     } finally {
       setCancellingId(null);
     }

frontend/app/booking/page.tsx

@@ -302,11 +302,7 @@ export default function BookingPage() {
         await fetchSlots(selectedDate);
       }
     } catch (err) {
-      if (err instanceof Error) {
-        setError(err.message);
-      } else {
-        setError("Failed to book appointment");
-      }
+      setError(err instanceof Error ? err.message : "Failed to book appointment");
     } finally {
       setIsBooking(false);
     }