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/mailers/application_mailer.rb

@@ -0,0 +1,4 @@
+class ApplicationMailer < ActionMailer::Base
+  default from: "from@example.com"
+  layout "mailer"
+end