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/architecture.adoc

@@ -40,7 +40,16 @@ regular page.
   local-only, not exposed on the network).
 
 | `public/`, `views/`
-| Frontend assets and Sinatra templates. The map view uses Leaflet.
+| Frontend assets and Sinatra templates. The map view uses MapLibre GL
+  JS, rendering vector tiles.
+
+| `public/vendor/`
+| Third-party frontend libraries, vendored (not loaded from a CDN) so
+  the app also works with no internet access. See
+  `docs/third-party-notices.adoc`.
+
+| `data/`
+| Local, gitignored data files, e.g. `offline.pmtiles`.
 
 | `src-tauri/`
 | The Tauri (Rust) desktop shell.
@@ -74,6 +83,80 @@ instead of relying on the system Ruby. That decision — and the
 resulting sidecar build step — is not implemented yet and should be
 revisited before any real distribution build.
 
+== Offline maps
+
+Map data is vector, not raster: the app ships
+https://maplibre.org[MapLibre GL JS] (vendored, see below) and the map
+view (`public/js/map.js`) offers two styles:
+
+* *Online* — a full hosted vector style,
+  https://tiles.openfreemap.org/styles/liberty[OpenFreeMap "liberty"],
+  requires internet access.
+* *Offline* — built at runtime from a local
+  https://github.com/protomaps/PMTiles[PMTiles] archive at
+  `data/offline.pmtiles`, served as-is (see below) at
+  `/tiles/offline.pmtiles`, decoded client-side by
+  https://github.com/protomaps/PMTiles[pmtiles.js] registered as a
+  `pmtiles://` protocol handler, and styled with base layers from
+  https://github.com/protomaps/basemaps[protomaps-themes-base].
+
+`GET /api/offline-tiles` reports whether `data/offline.pmtiles` is
+present (`{"available": true|false}`); the frontend uses it once, on
+load, to default to the offline style when local tile data exists —
+otherwise it defaults to the online style. Either way, the layer
+toggle in the top-left of the map lets the user switch manually.
+
+PMTiles is a single, self-indexed file, which is why this needs no
+tile-decoding logic on the server: `app.rb`'s `/tiles/offline.pmtiles`
+route (`send_file`, which Sinatra serves with HTTP range-request
+support) just returns byte ranges of the file; pmtiles.js in the
+browser reads the archive's own directory and requests only the bytes
+it needs for the tiles on screen.
+
+=== Known limitation: no offline text labels
+
+The offline style only renders `protomaps-themes-base`'s non-label
+layers (roads, buildings, land use, water — no place names or street
+labels). Label rendering in MapLibre needs a "glyphs" server serving
+font PBF ranges, which would mean vendoring a font glyph set locally
+too; that has not been done, to keep the initial scope manageable.
+Revisit this if labelled offline maps turn out to matter in practice.
+
+No PMTiles file ships with the repository — it is expected to be
+supplied locally (see `docs/development.adoc`) and stays gitignored
+because such extracts are typically large binary files.
+
+== Weather radar overlay
+
+`public/js/radar.js` adds a "Radar" map control that overlays recent
+precipitation radar from https://www.rainviewer.com[RainViewer]'s free
+public API (no API key required). Enabling it:
+
+. Fetches `https://api.rainviewer.com/public/weather-maps.json`,
+  which lists RainViewer's past radar frames (fixed 10-minute
+  cadence, at most 13 covering the last 120 minutes).
+. Takes the most recent 7 (last ~60 minutes — the same default as
+  the https://github.com/jpettitt/weather-radar-card[Home Assistant
+  weather-radar-card], for a consistent feel with that setup) and
+  adds one MapLibre raster source/layer per frame, all pre-loaded but
+  only the current frame's `raster-opacity` non-zero.
+. Animates by toggling that opacity across frames on an interval
+  (play/pause + a scrub slider), rather than swapping tiles in and out
+  — avoids reloading tiles on every step.
+
+Tile URL: `{host}{frame.path}/256/{z}/{x}/{y}/2/1_0.png` — `2` is
+RainViewer's "Universal Blue" color scheme, `1` enables their
+smoothing, `0` disables the snow overlay. `host` and each frame's
+`path` come from the API response.
+
+Since `map.setStyle()` (switching Online/Offline) tears down all
+custom sources and layers, the control listens for `style.load` and
+re-adds the radar layers if the overlay was on.
+
+This is a live, online-only overlay — it always needs internet access
+and is unrelated to the offline PMTiles basemap above. Per RainViewer's
+terms, the API is free for personal/non-commercial use.
+
 == Networking
 
 The Sinatra server only binds to `127.0.0.1`. It is not meant to be