nix build's staticFilesCompiledByMake phase calls make static/prod.css,
which depends on static/app.css. Added a minimal pre-built static/app.css
(satisfies Makefile dep without needing Tailwind in build sandbox) and a
Makefile rule to regenerate it via Tailwind in devenv.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>