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/environment.rb

@@ -0,0 +1,5 @@
+# Load the Rails application.
+require_relative "application"
+
+# Initialize the Rails application.
+Rails.application.initialize!