fix(root): show notice when JavaScript is disabled

[sylvesterkaczmarek]

Summary

Add a site-wide notice for users browsing the guidance site with JavaScript disabled.

The notice is rendered within the SSR layout using a native <noscript> element, so the existing page remains visible below it when JavaScript is unavailable.

Change

• render the notice above the existing page navigation within the SSR layout;
• explain that JavaScript is required for the best experience and full site functionality;
• style the notice using existing spacing and typography tokens and system colours.

Closes #186

[GCHQ-Developer-530]

Please can you rebase this pull request?

[sylvesterkaczmarek]

Rebased onto the latest develop. Thanks!

[GCHQ-Developer-299]

From my understanding, this ticket requests that an alert be added to every page that warns users about disabling javascript, while still displaying the page below.

This full replacement of the site with a message does not fulfil the requirements

[CLAassistant]

All committers have signed the CLA.

[sylvesterkaczmarek]

Thanks for the review. I’ve updated this so the <noscript> notice is rendered within the SSR layout above navigation, while the existing page remains visible below it. I’ve also rebased onto current develop and rerun formatting, type checks, lint, tests, and the production build. Happy to adjust anything else.