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.

views/layout.erb

@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>MapTool</title>
+  <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css">
+  <link rel="stylesheet" href="/css/style.css">
+</head>
+<body>
+  <%= yield %>
+  <script src="https://unpkg.com/leaflet@1.9.4/dist/leaflet.js"></script>
+  <script src="/js/map.js"></script>
+</body>
+</html>