update: set navbar and page header to sticky on scroll

[AdamJ]

Description of changes

Update the navigation and page headers to stick to the top of the page when a user scrolls.

Related to the following:

• https://github.com/ComponentAssemblySystems/CASimAI/issues/51
• https://github.com/ComponentAssemblySystems/caswow.library/commit/3af5e2790ea64708654988ad7a4498acacfb1ecb

PR checklist

Please check the boxes that apply before requesting review.

• I ran the site locally (pnpm show) and verified the changed pages
• I ran the production build (pnpm css && pnpm build) and checked docs/ output
• I updated or copied vendor assets using the package.json copy scripts if required (pnpm copy-css|copy-js|copy-fa|copy-cas)
• If this change depends on caswow.library, I documented whether it was linked with npm link or included in CI
• I verified there are no uncaught exceptions in the browser console (common: dialog script in base.njk)
• I added notes in this PR about any assumptions made (missing vendor files, local links, env variables)

Optional guidance for reviewers:

• For layout or JS changes, preview the affected page(s) in docs/ after a prod build.
• If vendor files were updated, confirm dependencies/ and docs/ contain the expected updated files.