fix: build CSS with Hugo's native TailwindCSS instead of PostCSS

hugo server/build was failing with "binary postcss is not a Node.js
script" — pnpm's default node_modules/.bin shims are shell wrappers, not
plain Node scripts, which Hugo's exec-security check rejects. Switches
head.html from css.PostCSS to css.TailwindCSS, drops the now-unused
PostCSS toolchain, and adds pnpm-workspace.yaml's
preferSymlinkedExecutables so pnpm emits real symlinks Hugo can exec
(same fix already in use on roux).

Also updates the Dockerfile's build stage to hugomods/hugo:debian-node,
which bundles the matching Hugo Extended + Node versions, keeping ffmpeg
for the video-compression step.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012E2mmywuZyXmPBgSDtNdTL

Dockerfile

@@ -1,16 +1,20 @@
 # Stage 1: Build
-FROM node:22-alpine AS builder
+# hugomods image bundles Hugo Extended + Node at matching, tested versions —
+# Hugo's css.TailwindCSS execs node_modules/.bin/tailwindcss during the build,
+# and pnpm-workspace.yaml's preferSymlinkedExecutables makes that a plain
+# symlink Hugo's exec-security check accepts (see layouts/partials/head.html).
+FROM hugomods/hugo:debian-node-0.165.0 AS builder
 
-# Install Hugo + FFmpeg (video compression)
-RUN apk add --no-cache hugo ffmpeg git
+# Install FFmpeg (video compression)
+RUN apt-get update && apt-get install -y --no-install-recommends ffmpeg && rm -rf /var/lib/apt/lists/*
 
 # Install pnpm
-RUN corepack enable && corepack prepare pnpm@latest --activate
+RUN npm install -g pnpm
 
 WORKDIR /app
 
 # Copy package files first (for layer caching)
-COPY package.json pnpm-lock.yaml ./
+COPY package.json pnpm-lock.yaml pnpm-workspace.yaml ./
 RUN pnpm install --frozen-lockfile
 
 # Copy source files