[GF-FE-13] Add promo and discount code input field to the booking payment flow

[yusuftomilola]

Problem

The booking flow has no promo or discount code input. Members who have been given a promotional code (e.g. for a corporate rate or referral reward) have no way to apply it at checkout. This makes promo codes backend-only with no member-facing surface.

Proposed Solution

Extend the booking wizard in frontend/app/bookings/new/page.tsx to include a promo code field in the payment step.

Changes:

1. Add a "Have a promo code?" collapsible section before the payment button
2. Inside: text input for the code + an Apply button
3. On clicking Apply, call POST /promo-codes/validate with { code, workspaceId, bookingAmount }
4. On success: show a green confirmation banner — "₦X discount applied" or "X% off" — and update the displayed total
5. On error: show inline error message ("Code expired", "Code not valid for this workspace type", etc.)
6. Show a Remove link to clear the applied code and restore the original total
7. Pass the validated promo code to the booking creation/payment request

Acceptance Criteria

• Promo code input and Apply button added to the payment step of the booking flow
• POST /promo-codes/validate is called when Apply is clicked
• Discount and updated total are shown on valid code
• Inline error is shown for invalid, expired, or already-used codes
• Remove clears the code and restores the original total
• The promo code is included in the payment initialisation request

Depends on: [GF-BE-09] Build promo codes and discount backend module

[barry01-hash]

My chief, i will love to work on this issue

[khanavi272-spec]

@khanavi272-spec has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @khanavi272-spec to this issue.

[0xElyte]

@0xElyte has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I'd love to resolve this issue fast

ℹ️ Repo Maintainers: To accept this application, review their application or assign @0xElyte to this issue.

[Michael997glitch]

@Michael997glitch has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I’ve resolved similar problems in the past and am confident I can implement a clean, efficient fix quickly,if am given the opportunity.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Michael997glitch to this issue.

[directorfloo]

@directorfloo has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i will love to work on this issue professionally ,please assign it to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @directorfloo to this issue.

[dzekojohn4]

@dzekojohn4 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, can I take this?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @dzekojohn4 to this issue.

[drips-wave]

Congratulations, @dzekojohn4! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @dzekojohn4: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[grantfox-oss]

🦊 GrantFox — @dzekojohn4 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #1108)
2. Your PR will be reviewed by the DistinctCodes maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@dzekojohn4's PR #1319 was approved and merged by @yusuftomilola.

🏆 @dzekojohn4: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (366 total points). Track your full progress on GrantFox.

👏 Great work, @dzekojohn4! Keep contributing to DistinctCodes.