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.

README.adoc

@@ -10,8 +10,10 @@ The project's shape is still open — this is the initial scaffold.
 
 == Status
 
-Early scaffold. The web application serves a basic Leaflet map; the
-desktop shell boots the same server inside a native window.
+Early scaffold. The web application serves a vector map (MapLibre GL
+JS), with an online style and an optional offline style backed by a
+local PMTiles file; the desktop shell boots the same server inside a
+native window.
 
 == Requirements
 
@@ -33,3 +35,4 @@ Then open http://127.0.0.1:4567 in a browser.
 
 * link:docs/architecture.adoc[Architecture]
 * link:docs/development.adoc[Development guide]
+* link:docs/third-party-notices.adoc[Third-party notices]