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.

.gitignore

@@ -8,6 +8,10 @@
 # Rust / Tauri
 /src-tauri/target/
 
+# Offline map tile data (large binary; obtained/generated locally,
+# see docs/development.adoc)
+/data/*.pmtiles
+
 # Editor / OS
 .DS_Store
 *.swp