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

package.json

@@ -11,10 +11,7 @@
     "format:check": "prettier --check \"layouts/**/*.html\" \"assets/**/*.css\" \"assets/**/*.js\" \"content/**/*.md\" \"*.toml\""
   },
   "devDependencies": {
-    "@tailwindcss/postcss": "^4.0.0",
-    "autoprefixer": "^10.4.0",
-    "postcss": "^8.5.0",
-    "postcss-cli": "^11.0.0",
+    "@tailwindcss/cli": "^4.0.0",
     "prettier": "^3.8.3",
     "prettier-plugin-go-template": "^0.0.15",
     "prettier-plugin-toml": "^2.0.6",