Initial commit: Firewood Stack calculator with CI/CD

Static web app (nginx) with cord calculation, wood species/seasoning
adjustments, season planning, and month-by-month coverage timeline.
GitHub Actions workflow builds and pushes container to ghcr.io on push to main.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,3 @@
+FROM nginx:alpine
+COPY index.html /usr/share/nginx/html/index.html
+EXPOSE 80