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/app.d.ts

@@ -13,6 +13,7 @@ declare global {
 		__walkInCountdownInterval?: ReturnType<typeof setInterval>;
 		__walkInModalCountdownInterval?: ReturnType<typeof setInterval>;
 		__bookingCreateCountdownInterval?: ReturnType<typeof setInterval>;
+		__bookingFlowCountdownInterval?: ReturnType<typeof setInterval> | null;
 	}
 }