feat: make SITE_URL and SITE_DOMAIN dynamic via .env and docker compose

- Create .env and .env.example with SITE_DOMAIN=riverrooks.dev and SITE_URL=https://riverrooks.dev
- Migrate src/_data/site.json to src/_data/site.js reading environment variables dynamically
- Update subdomains across Forgejo, CyberChef, Stirling PDF, and Uptime Kuma to inherit site.domain
- Update docker-compose.yml and Dockerfile to accept dynamic SITE_URL and SITE_DOMAIN build args
- Add .env to .gitignore

Dockerfile

@@ -3,6 +3,12 @@ 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=$SITE_URL
+ENV SITE_DOMAIN=$SITE_DOMAIN
+
 # Copy dependency definitions and install
 COPY package*.json ./
 RUN npm ci