Replaces the PostCSS + autoprefixer toolchain with Hugo's native
css.TailwindCSS pipe (via @tailwindcss/cli), matching the setup already
in use on pivoine.art. Dockerfile now builds on hugomods/hugo with a
matched Hugo/Node version instead of manually installing Hugo onto
node:22-alpine.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3xLyuoN6uzfCYBo8kVoSn
