feat(bookings): promo code input on payment step with /promo-codes/validate

[dzekojohn4]

Closes #1108
Closes #1106
Closes #1107
Closes #1109
Adds a promo code input to the Payment step of the booking wizard (frontend/components/bookings/BookingForm.tsx) so members can apply referral or corporate promo codes at checkout.

What changed

1. Moved the promo-code input out of step 0 (Details) and into step 2 (Payment), where it sits before the Pay button per the issue spec.
2. Fixed the validate payload to match the backend ValidatePromoCodeDto (which requires workspaceId + bookingAmount):

   POST /promo-codes/validate
   { code, workspaceId, bookingAmount: totalAmount }
   

3. Compute the discount kobo client-side because the backend returns discountValue (raw: percent for PERCENTAGE, kobo for FIXED) plus finalAmount. We derive discountAmountKobo from discountType so we can show "₦X off" in the UI; the finalAmount from the server is what the Paystack amount uses.
4. Remove link clears appliedPromo + promoCodeInput and restores the original total.
5. Pay button's displayed amount now reflects appliedPromo.finalAmountKobo when a code is applied.

Acceptance criteria

• Promo code input and Apply button on the payment step (before the Pay button)
• POST /promo-codes/validate called on Apply with the documented DTO
• Discount and updated total shown on a valid code (green confirmation banner)
• Inline error shown for invalid / expired / already-used codes
• Remove clears the code and restores the original total
• Pass promoCode to the booking creation / payment initialisation request — the backend already records appliedPromoCodeId server-side via PromoCodesService.applyToBooking; the wizard does not yet forward a promoCodeId because the existing useCreateBooking/useInitializePayment hooks do not accept one. Wiring that server-side hook is best done in a follow-up PR.

Note

Backend, deps and frontend build were not exercised in the sandbox where this PR was authored (no frontend/node_modules); please run cd frontend && npm install && npm run lint && npm run build before reviewing.

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