feat(GAT-9201): Add NHS SDE access stepper scaffold

[philreekshdr]

AI disclaimer - CLAUDE was used for this. The branch strategy, staged-PR split and scope boundaries were planned manually; the component build, translations and unit tests were implemented by CLAUDE against that plan.

Screenshots / videos (if relevant)

Describe your changes

Replaces the old NHS SDE cells on the Cohort Discovery cover page with a new 5-step NhsSdeAccessStepper panel. Step 1 ("Existing Cohort Discovery Access") is locked with a padlock until the user's Cohort Discovery request is APPROVED, at which point it completes and reveals a local "Apply for NHS…" opt-in. Steps 2–5 are titles-only scaffolding for now — the interactive validation flow (open form, consent, indicate, status badges) lands in GAT-9203, which stacks on this branch. When the isNhsSdeApplicationsEnabled flag is off, the panel shows the existing pilot text instead of the steps.

Issue ticket link

https://hdruk.atlassian.net/browse/GAT-9201

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"

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