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

.gitattributes

@@ -0,0 +1,9 @@
+# See https://git-scm.com/docs/gitattributes for more about git attribute files.
+
+# Mark the database schema as having been generated.
+db/schema.rb linguist-generated
+
+# Mark any vendored files as having been vendored.
+vendor/* linguist-vendored
+config/credentials/*.yml.enc diff=rails_credentials
+config/credentials.yml.enc diff=rails_credentials