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

app/controllers/pages_controller.rb

@@ -0,0 +1,4 @@
+class PagesController < ApplicationController
+  def home
+  end
+end