fix: BookingConfirmPage date format + guest confirmation email on payment

- Fix date display: slice ISO timestamp to first 10 chars before splitting
- Add sendBookingConfirmedEmail() to email.ts — sends green styled HTML email
  with booking details (dates, amount) on payment.succeeded webhook
- Webhook now sends confirmation email to guest email if present

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/BookingConfirmPage.tsx

@@ -19,7 +19,7 @@ interface OnlineBookingStatus {
 }
 
 function formatDate(d: string) {
-  const [y, m, day] = d.split('-')
+  const [y, m, day] = d.slice(0, 10).split('-')
   return `${day}.${m}.${y}`
 }