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.

Gemfile

@@ -0,0 +1,9 @@
+source "https://rubygems.org"
+
+gem "sinatra", "~> 4.2"
+gem "sinatra-contrib", "~> 4.2"
+gem "puma", "~> 8.0"
+
+group :development do
+  gem "rerun"
+end