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.

public/css/style.css

@@ -0,0 +1,9 @@
+html, body {
+  height: 100%;
+  margin: 0;
+}
+
+#map {
+  height: 100%;
+  width: 100%;
+}