fix: move BookingFlow window type to app.d.ts and fix authHeaders typing

Move __bookingFlowCountdownInterval from inline declare global in BookingFlow.svelte to global app.d.ts. Fix authHeaders type from {} to undefined to satisfy TypeScript.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/lib/components/booking/BookingFlow.svelte

@@ -27,12 +27,6 @@
 	import { onDestroy } from 'svelte';
 	import { SvelteDate } from 'svelte/reactivity';
 
-	declare global {
-		interface Window {
-			__bookingFlowCountdownInterval?: ReturnType<typeof setInterval> | null;
-		}
-	}
-
 	// Components
 	import BookingActions from '$lib/components/booking/BookingActions.svelte';
 	import BookingSummary from '$lib/components/booking/BookingSummary.svelte';
@@ -688,7 +682,7 @@
 		loadingAvailableHours = true;
 
 		try {
-			const authHeaders = authStore.currentToken ? { Authorization: `Bearer ${authStore.currentToken}` } : {};
+			const authHeaders: RequestInit['headers'] = authStore.currentToken ? { Authorization: `Bearer ${authStore.currentToken}` } : undefined;
 			const [whRes, ahRes] = await Promise.all([
 				fetch(`/api/scheduling/working-hours?start=${startStr}&end=${endStr}`, { headers: authHeaders }),
 				fetch(`/api/scheduling/available-hours?start=${startStr}&end=${endStr}`, { headers: authHeaders })
@@ -782,7 +776,7 @@
 			const startStr = startOfMonth.toString();
 			const endStr = endOfMonth.toString();
 
-			const authHeaders = authStore.currentToken ? { Authorization: `Bearer ${authStore.currentToken}` } : {};
+			const authHeaders: RequestInit['headers'] = authStore.currentToken ? { Authorization: `Bearer ${authStore.currentToken}` } : undefined;
 
 			// Fetch working hours
 			const workingHoursResponse = await fetch(