Set up Rails 8 application foundation

- Generate Rails 8.1 app with PostgreSQL and TailwindCSS
- Add docker-compose.yml (postgres:17) for local database
- Configure database.yml to connect to Dockerized Postgres via env vars
- Add Pages#home root route, application layout, and navbar partial

public/icon.svg

@@ -0,0 +1,3 @@
+<svg width="512" height="512" xmlns="http://www.w3.org/2000/svg">
+  <circle cx="256" cy="256" r="256" fill="red"/>
+</svg>