CCM-9916: preload fonts with cors

[harrim91]

Description

• Adds font preloads with an explicit CORS directive, which allows them to be loaded by Safari.

Context

When opening notify.nhs.uk in Safari, the site failed to load fonts served from https://assets.nhs.uk.
The browser console showed:

Failed to load resource: Cancelled load to https://assets.nhs.uk/fonts/FrutigerLTW01-55Roman.woff2 because it violates the resource's Cross-Origin-Resource-Policy response header.

The site’s HTML was being served with:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

These headers enable cross-origin isolation. Under this policy, cross-origin assets must either:

• be fetched with CORS
• be served with a Cross-Origin-Resource-Policy (CORP) header of cross-origin or same-site.

Font fetches were happening from within a @font-face block in the nhsuk-frontend css.
Safari was doing these fetches as no-CORS requests which caused them to be rejected.

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.

[emmagifford]

Michael explained the context to me over Teams. We both had issues getting the codespace to load, so Michael's added screenshots of the site running locally on his machine. I can see that the fonts appear to load correctly in the Safari browser.

Happy to approve - well done!