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

config.ru

@@ -0,0 +1,6 @@
+# This file is used by Rack-based servers to start the application.
+
+require_relative "config/environment"
+
+run Rails.application
+Rails.application.load_server