build: switch to native Hugo TailwindCSS pipeline, align Dockerfile with pivoine.art

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

layouts/partials/head.html

@@ -90,8 +90,8 @@
   />
 </noscript>
 
-<!-- CSS via Hugo Pipes + PostCSS + Tailwind v4 -->
-{{- $css := resources.Get "css/main.css" | css.PostCSS -}}
+<!-- CSS via Hugo Pipes + Tailwind v4 (native css.TailwindCSS, no PostCSS) -->
+{{- $css := resources.Get "css/main.css" | css.TailwindCSS -}}
 {{- if eq hugo.Environment "production" -}}
   {{- $css = $css | minify | fingerprint "sha256" -}}
 {{- end -}}