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.

docs/third-party-notices.adoc

@@ -0,0 +1,33 @@
+= Third-party notices
+:toc:
+:toclevels: 1
+
+Frontend libraries vendored under `public/vendor/` instead of loaded
+from a CDN, so the app keeps working with no internet access. All are
+BSD-3-Clause licensed.
+
+[cols="1,1,2"]
+|===
+| Library | Version | Source
+
+| MapLibre GL JS
+| 6.6.0
+| https://github.com/maplibre/maplibre-gl-js
+
+| pmtiles (pmtiles.js)
+| 4.5.0
+| https://github.com/protomaps/PMTiles
+
+| protomaps-themes-base
+| 4.5.0
+| https://github.com/protomaps/basemaps
+|===
+
+The online map style (not vendored, fetched at runtime when the
+"Online" layer is selected) is
+https://tiles.openfreemap.org/styles/liberty[OpenFreeMap "liberty"],
+built from https://www.openstreetmap.org[OpenStreetMap] data.
+
+To update a vendored library: download its package (e.g.
+`npm pack <name>@<version>`), replace the corresponding files under
+`public/vendor/<name>/`, and update the version in this file.