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/development.adoc

@@ -18,6 +18,38 @@ via the `rerun` gem instead:
 bundle exec rerun -- bundle exec puma -C config/puma.rb
 ----
 
+TIP: Browsers apply heuristic caching to static files even without
+explicit cache headers, so an edit to `public/` (or `views/`) can
+appear not to have taken effect on the next reload. Force a real
+reload (e.g. Ctrl/Cmd+Shift+R) if that happens.
+
+== Offline map data
+
+To enable the offline style, place a PMTiles file at
+`data/offline.pmtiles` (see `docs/architecture.adoc` for how it is
+served and styled). It is not part of the repository — obtain or
+build one locally:
+
+* The simplest source is a pre-built extract from
+  https://app.protomaps.com[protomaps.com's extract builder] — pick a
+  region and download it as `.pmtiles`. Its schema is what
+  `protomaps-themes-base` (the styling library used for the offline
+  style) expects.
+* To convert some other vector tile source (e.g. an `.mbtiles` file)
+  into PMTiles, use the
+  https://github.com/protomaps/go-pmtiles[go-pmtiles] CLI:
+  `pmtiles convert in.mbtiles data/offline.pmtiles`.
+
+Once the file is in place, `GET /api/offline-tiles` reports
+`available: true` and the map defaults to the offline style.
+
+This is picked up automatically for `cargo tauri dev` (it just runs
+the Sinatra app from the project root). To ship it inside a *packaged*
+build, add it to `bundle.resources` in `src-tauri/tauri.conf.json`:
+`"../data/offline.pmtiles": "data/offline.pmtiles"`. Not added by
+default, since these files can be large and are picked per
+deployment.
+
 == Desktop shell (Tauri)
 
 === Prerequisites