feat(GAT-9203): Add NHS SDE registration validation flow - #1603

AI disclaimer - CLAUDE was used for this. The stacked-branch strategy, NHS status/flow model and scope split were planned manually; the step 2–5 component logic, central status consts, copy and unit tests were implemented by CLAUDE and reviewed by me.

Screenshots / videos (if relevant)

Describe your changes

Fills in steps 2–5 of the NHS SDE access stepper scaffolded in GAT-9201. Step 2 opens the external NHS SDE registration form (firing the request_nhse_access request → IN PROCESS) and reveals a "confirm completed" action; step 3 takes consent and, via a confirmation modal, submits the indicate-access request (→ APPROVAL REQUESTED); steps 4–5 show the review/decision terminal states. Adds an activeStep progression and status badges (Awaiting Action, Pending, Approved and negative states) driven by nhseSdeRequestStatus, and lifts the NHS/cohort status strings into central NHS_SDE_STATUS/COHORT_STATUS consts. Also renames the user-facing "validation" copy to "registration" to match the renamed service, adds variant/label props to the two NHS buttons, and fixes step-title muting so completed steps stay dark (only future steps grey out).

Issue ticket link

https://hdruk.atlassian.net/browse/GAT-9203

Checklist before requesting a review

• I have performed a self-review of my code
• I have added appropriate unit tests / e2e tests (where relevant)
• I have created mocks for api endpoints (where appropriate)
• The interface is responsive (where ticket is relevant)
• The interface is at least AA (where ticket is relevant)
• Commits are described as "(chore|fix|feature|test|maintenance): description"