fix(next): only set payload theme preference headers within '/admin' namespace

[lynndylanhurley]

When these headers are set globally, they interfere with the lighthouse CI tool. It forces a redirect that adds 600-800ms to the total load time on mobile.

By only setting the headers for the /admin section of the site, which is where they are used, it resolves the problem for other sections of the site where performance is critical.

Description

• I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Steps to test

1. Pull down the latest version of the payload 3.0 demo
2. Run the site in prod mode: pnpm build && pnpm start
3. Run a lighthouse audit using the cli tool: npx lighthouse@latest http://localhost:3000 --view --chrome-flags="--headless" --verbose
4. Observe this line in the report:
   

Steps to confirm the fix

1. Open the file: node_modules/@payloadcms/next/dist/withPayload.js
2. Apply the change from this PR by changing source: '/:path*' to source: '/admin/:path*'
3. Re-run build and run the site: pnpm build && pnpm start
4. Re-run the lighthouse check: npx lighthouse@latest http://localhost:3000 --view --chrome-flags="--headless" --verbose
5. Observe that the redirect perfomance penalty has been resolved

[lynndylanhurley]

Checking in on this. Is there anything more that needs to be done for this to be merge-able?