Switch to vector maps (MapLibre GL JS + PMTiles) and add RainViewer radar

Replace Leaflet/raster tiles with MapLibre GL JS: an online OpenFreeMap
style and an offline style built from a local PMTiles archive via
pmtiles.js and protomaps-themes-base (no glyphs/labels offline by
design, see docs/architecture.adoc). Frontend libraries are vendored
locally rather than loaded from a CDN so the app keeps working with no
internet access.

Also add an animated RainViewer precipitation radar overlay (toggle,
play/pause, scrub slider), matching the default time window of the
weather-radar-card Home Assistant integration.

views/layout.erb

@@ -4,12 +4,13 @@
   <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="/vendor/maplibre-gl/maplibre-gl.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>
+  <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>
 </body>
 </html>