feat: throw explicit error when SITE_DOMAIN is unset

- Throw fatal configuration error in src/_data/site.js if SITE_DOMAIN is missing
- Require SITE_DOMAIN in docker-compose.yml without default fallback
- Require SITE_DOMAIN ARG in Dockerfile with no default fallback

Dockerfile

@@ -3,11 +3,11 @@ FROM node:22-alpine AS builder
 
 WORKDIR /app
 
-# Dynamic build arguments for domain & full URL
-ARG SITE_URL=https://riverrooks.dev
-ARG SITE_DOMAIN=riverrooks.dev
-ENV SITE_URL=$SITE_URL
+# Dynamic build arguments for domain & full URL (required, no fallback)
+ARG SITE_DOMAIN
+ARG SITE_URL
 ENV SITE_DOMAIN=$SITE_DOMAIN
+ENV SITE_URL=$SITE_URL
 
 # Copy dependency definitions and install
 COPY package*.json ./