Initial scaffold: Sinatra map app with Tauri desktop shell

Sinatra backend serving a basic Leaflet map, wrapped in a Tauri
desktop shell that starts the same server in a native window.

.gitignore

@@ -0,0 +1,16 @@
+# Ruby / Bundler
+/.bundle/
+/vendor/bundle/
+*.gem
+/log/
+/tmp/
+
+# Rust / Tauri
+/src-tauri/target/
+
+# Editor / OS
+.DS_Store
+*.swp
+
+# Local tooling settings
+.claude/settings.local.json