Play Framework

fix: strip play-tailwind.css reference from default error templates (PF-77)

The default 404/500 error templates referenced @{'/public/stylesheets/play-tailwind.css'}
via Play's reverse-route lookup. That CSS asset only ships with apps generated via
`play new --frontend nuxt` — plain Play apps don't have it. When such an app produced
a 404 or 500, rendering the default error template threw NoRouteFoundException, which
the error handler converted into a 500 with body "Compilation error: No route found ..."
— masking the original 404 entirely and giving wrong status codes for genuine 500s.

Stripped the external CSS link tag and the Tailwind utility classes from the body of
both framework/templates/errors/404.html and 500.html. The dev/prod template branches
themselves are unchanged. Error pages no longer pull in any external assets.

Removed the now-redundant testapp overrides at
framework/test-src/integration/testapp/app/views/errors/{404,500}.html that PF-5
introduced as a workaround. The integration suite now exercises the framework
defaults directly.

Verified: ant integration-test passes, including SecurityHeadersFunctionalTest cases
notFoundResponseHasSecurityHeaders (HTTP 404) and serverErrorResponseHasSecurityHeaders
(HTTP 500) against the framework default templates.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>