fix(GRO-2105): include serviceId in BookingFlow/RescheduleFlow availability call

Both portal flows were calling /api/book/availability with only the date
param, causing the API to return 400 'serviceId and date are required'.
The handler then .map()'d the {error:...} object, throwing TypeError and
wiping <div id="root">.

- Add fetchAvailability helper that validates r.ok and Array.isArray(data)
- RescheduleFlow: pass appt.serviceId
- BookingFlow: pass selectedServices[0]?.id
- Surface API error message ('Failed to load time slots' or the server's
  error string) instead of crashing
- Add a cancelled guard to prevent stale responses from overwriting state

GRO-1804 / TC-WEB-5.12.5 through 5.12.11 unblocked.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/portal/sections/Appointments.tsx

@@ -2,6 +2,35 @@ import React, { useState, useEffect } from 'react';
 import { Calendar, Clock, Plus, ChevronRight, ChevronDown, Loader2 } from 'lucide-react';
 import { ANALYTICS_EVENTS, fireAnalyticsEvent } from '../../lib/analytics';
 
+// ─── Availability fetch helper ───────────────────────────────────────────────
+// Returns ISO startTime strings for the given service/date, or an error message.
+// Validates HTTP status and that the body is actually an array — the API
+// responds with `{error: "..."}` on 4xx, and we must not treat that as slots.
+const AVAILABILITY_ERROR_MESSAGE = 'Failed to load time slots';
+
+async function fetchAvailability(
+  params: { serviceId: string; date: string },
+  sessionId: string | null,
+): Promise<{ times: string[]; error: string | null }> {
+  const url = `/api/book/availability?${new URLSearchParams(params).toString()}`;
+  const headers: Record<string, string> = {};
+  if (sessionId) headers['X-Impersonation-Session-Id'] = sessionId;
+  try {
+    const res = await fetch(url, { headers });
+    if (!res.ok) {
+      const body = (await res.json().catch(() => ({}))) as { error?: string };
+      return { times: [], error: body.error ?? `${AVAILABILITY_ERROR_MESSAGE} (HTTP ${res.status})` };
+    }
+    const data: unknown = await res.json();
+    if (!Array.isArray(data)) {
+      return { times: [], error: AVAILABILITY_ERROR_MESSAGE };
+    }
+    return { times: data as string[], error: null };
+  } catch {
+    return { times: [], error: AVAILABILITY_ERROR_MESSAGE };
+  }
+}
+
 export interface Appointment {
   id: string;
   petId: string;
@@ -595,19 +624,29 @@ export function RescheduleFlow({
   useEffect(() => {
     if (!selectedDate || !sessionId) {
       setAvailableTimes([]);
+      setSlotsError(null);
       return;
     }
-    const params = new URLSearchParams({ date: selectedDate });
+    if (!appt.serviceId) {
+      setAvailableTimes([]);
+      setSlotsError('Failed to load time slots');
+      return;
+    }
+    let cancelled = false;
     setSlotsLoading(true);
     setSlotsError(null);
-    fetch(`/api/book/availability?${params.toString()}`, {
-      headers: { "X-Impersonation-Session-Id": sessionId ?? "" },
-    })
-      .then((r) => r.json() as Promise<string[]>)
-      .then(setAvailableTimes)
-      .catch(() => setSlotsError('Failed to load time slots'))
-      .finally(() => setSlotsLoading(false));
-  }, [selectedDate, sessionId]);
+    fetchAvailability({ serviceId: appt.serviceId, date: selectedDate }, sessionId).then(
+      ({ times, error }) => {
+        if (cancelled) return;
+        setAvailableTimes(times);
+        setSlotsError(error);
+        setSlotsLoading(false);
+      },
+    );
+    return () => {
+      cancelled = true;
+    };
+  }, [selectedDate, sessionId, appt.serviceId]);
 
   async function handleSubmit() {
     if (!selectedDate || !selectedTime) return;
@@ -766,19 +805,30 @@ function BookingFlow({ onClose, sessionId }: BookingFlowProps) {
   useEffect(() => {
     if (!selectedDate || !sessionId) {
       setAvailableTimes([]);
+      setSlotsError(null);
       return;
     }
-    const params = new URLSearchParams({ date: selectedDate });
+    const serviceId = selectedServices[0]?.id;
+    if (!serviceId) {
+      setAvailableTimes([]);
+      setSlotsError('Failed to load time slots');
+      return;
+    }
+    let cancelled = false;
     setSlotsLoading(true);
     setSlotsError(null);
-    fetch(`/api/book/availability?${params.toString()}`, {
-      headers: { "X-Impersonation-Session-Id": sessionId ?? "" },
-    })
-      .then((r) => r.json() as Promise<string[]>)
-      .then(setAvailableTimes)
-      .catch(() => setSlotsError('Failed to load time slots'))
-      .finally(() => setSlotsLoading(false));
-  }, [selectedDate, sessionId]);
+    fetchAvailability({ serviceId, date: selectedDate }, sessionId).then(
+      ({ times, error }) => {
+        if (cancelled) return;
+        setAvailableTimes(times);
+        setSlotsError(error);
+        setSlotsLoading(false);
+      },
+    );
+    return () => {
+      cancelled = true;
+    };
+  }, [selectedDate, sessionId, selectedServices]);
 
   useEffect(() => {
     const fetchData = async () => {