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/application_controller.rb

@@ -0,0 +1,7 @@
+class ApplicationController < ActionController::Base
+  # Only allow modern browsers supporting webp images, web push, badges, import maps, CSS nesting, and CSS :has.
+  allow_browser versions: :modern
+
+  # Changes to the importmap will invalidate the etag for HTML responses
+  stale_when_importmap_changes
+end