Cache-bust app JS/CSS on each deploy

Browsers (and a caching proxy in front of production) were holding
onto /js and /css for hours, so a deploy could go unnoticed by an
already-open tab. A boot-time version query string on the entry
script/stylesheet, plus an import map remapping map.js's internal
module imports to the same versioned URLs, fixes that without editing
every JS file's import statements.

views/layout.erb

@@ -5,12 +5,30 @@
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>MapTool</title>
   <link rel="stylesheet" href="/vendor/maplibre-gl/maplibre-gl.css">
-  <link rel="stylesheet" href="/css/style.css">
+  <link rel="stylesheet" href="/css/style.css?v=<%= ASSET_VERSION %>">
 </head>
 <body>
   <%= yield %>
   <script src="/vendor/pmtiles/pmtiles.js"></script>
   <script src="/vendor/protomaps-themes-base/protomaps-themes-base.js"></script>
-  <script type="module" src="/js/map.js"></script>
+  <%# Vendored libs above don't change between deploys, so they're left
+      unversioned; app.js's own modules do, and map.js statically imports
+      them by bare path — an import map remaps those paths to a versioned
+      URL so editing e.g. gpx.js also busts its cache, without having to
+      touch every file's import statements. Must come before the module
+      script below: the importmap has to be parsed first. %>
+  <script type="importmap">
+    {
+      "imports": {
+        "/js/gnss.js": "/js/gnss.js?v=<%= ASSET_VERSION %>",
+        "/js/gpx.js": "/js/gpx.js?v=<%= ASSET_VERSION %>",
+        "/js/icons.js": "/js/icons.js?v=<%= ASSET_VERSION %>",
+        "/js/livetrack.js": "/js/livetrack.js?v=<%= ASSET_VERSION %>",
+        "/js/panels.js": "/js/panels.js?v=<%= ASSET_VERSION %>",
+        "/js/radar.js": "/js/radar.js?v=<%= ASSET_VERSION %>"
+      }
+    }
+  </script>
+  <script type="module" src="/js/map.js?v=<%= ASSET_VERSION %>"></script>
 </body>
 </html>