✨ allow custom voucher discount on admin order creation

[kernicPanel]

Purpose

The admin order creation form always attached a 100% discount voucher.
Admins need the ability to specify a custom discount (percentage or
fixed amount) when creating standalone orders.

Proposal

• Backend: AdminOrderCreateSerializer accepts optional
  discount_type (rate/amount) and discount_value fields.
  perform_create uses them to create the appropriate discount,
  defaulting to 100% when omitted.
• Frontend: A checkbox "Full discount (100%)" is checked by
  default. Unchecking it reveals a value input and a type selector
  (fixed amount by default, or percentage). Re-checking hides the
  fields and reverts to 100%.
• All messages are translated via defineMessages/intl.

[Copilot]

Pull request overview

This PR updates the admin standalone order creation flow to optionally create vouchers with a custom discount (percentage or fixed amount), instead of always forcing a 100% discount.

Changes:

• Backend: adds discount_type / discount_value to AdminOrderCreateSerializer and uses them in OrderViewSet.perform_create to create the right Discount.
• Frontend: extends the order creation form with a “Full discount (100%)” toggle that reveals discount inputs when unchecked, and sends the optional fields to the API.
• Tests: adds backend API tests plus frontend unit and Playwright E2E coverage for the new UI and payload behavior.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/backend/joanie/core/serializers/admin.py	Accepts optional discount_type/discount_value and enforces “both or none” presence.
src/backend/joanie/core/api/admin/__init__.py	Creates Discount as rate/amount based on the optional fields, otherwise defaults to full (rate=1).
src/backend/joanie/tests/core/api/admin/orders/test_create.py	Adds API tests for custom rate and amount discounts on order creation.
src/frontend/admin/src/services/api/models/Order.ts	Extends DTOOrderCreate with optional discount fields.
src/frontend/admin/src/components/templates/orders/form/OrderCreateForm.tsx	Adds UI and payload logic for full vs custom discounts, including translated labels/messages.
src/frontend/admin/src/components/templates/orders/form/OrderCreateForm.spec.tsx	Adds unit tests for toggling/visibility and request payload behavior.
src/frontend/admin/src/tests/orders/orders.test.e2e.ts	Adds Playwright E2E tests validating UI behavior and posted JSON payload fields.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.