feature: a mocked implementation of v1 UI done.

frontend/src/lib/treatmentSelection.ts

@@ -0,0 +1,25 @@
+import { isSameLocalCalendarDay } from '@/lib/appointmentTime';
+import type { TreatmentAppointment } from '@/types/treatment';
+
+/**
+ * For the selected calendar day: if it is today, pick the appointment whose time range contains now;
+ * otherwise pick the first appointment of that day. Returns null when there are no appointments.
+ */
+export function pickAutoAppointment(
+  appointments: TreatmentAppointment[],
+  selectedDay: Date,
+): string | null {
+  if (appointments.length === 0) return null;
+
+  const now = new Date();
+  if (isSameLocalCalendarDay(selectedDay, now)) {
+    const t = now.getTime();
+    for (const a of appointments) {
+      const s = new Date(a.startAt).getTime();
+      const e = new Date(a.endAt).getTime();
+      if (t >= s && t <= e) return a.id;
+    }
+  }
+
+  return appointments[0].id;
+}