Narrow scope to radar, and put the app on a real OpenStreetMap base map

Drops forecasts, lightning, the home-screen widget and advertising. What
remains is radar on a map, the official ARPA alert bulletin, and rain
notifications.

The base map is now OpenFreeMap's Positron style: real OpenStreetMap vector
tiles with no API key, no registration, no request limits and commercial use
permitted. Every other free tier — MapTiler, Stadia, Jawg, Thunderforest —
needs a key, which is a secret to manage, a quota to outgrow and a signup to
complete before anyone can build the project, and the map is the one thing the
app cannot work without. Positron rather than Liberty or Bright because the
radar overlay has to be the loudest thing on screen, and a desaturated grey
base is built to sit under data.

Its style JSON carries no `attribution` field, so MapLibre displays no credits
by itself. The app renders them from the region config instead: the two
mandatory credits, OpenStreetMap and OpenMapTiles, go in the always-visible
bar, and OpenFreeMap's own credit — optional by their terms — is listed on the
Sources screen with the rest. The bundled offline style is still reachable with
MAP_STYLE_URL=offline, and still claims no base map attribution, because
crediting OpenStreetMap while showing it would be a false claim.

Radar-DPC stays the source. ARPA Piemonte's own radar remains a disabled stub
for two reasons that belong to the project owner, not to the code: the
real-time access link is only issued by email, and the open-data page states
the data is "gratuiti" and nothing else. Free of charge is not a licence, and
rendering those volumes into frames served from a CDN is redistribution. Both
questions go in the same email. An earlier draft of the docs recorded ARPA
radar as CC BY 4.0; the source page does not support that, so the claim is
removed rather than carried forward.

The documentation is updated throughout rather than annotated: CLAUDE.md gains
an explicit scope boundary, data-sources drops MET Norway and ISTAT and gains
the base map, licenses records that free of charge is not a licence, privacy
loses the whole advertising section, and the roadmap is renumbered so the
backend worker is next — until it exists, DpcRadarSource has nothing to read.

licenses.md keeps Open-Meteo and Blitzortung listed as excluded even though the
features that would have used them are gone: both are non-commercial-only, ads
are a plausible future, and neither should be adopted on the grounds that there
are none today.

Verified: analyze clean, 130 tests passing, and on the emulator the radar
overlay sits correctly over Piedmont on real OSM tiles with Turin, Milan and
Genoa labelled, the age reads "Aggiornato 4 minuti fa", and the Sources screen
lists all five credits with their licences.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Nuvolari/app/lib/features/map/map_style.dart

@@ -5,9 +5,14 @@ import '../../core/region/region_config.dart';
 
 /// Which base map the app is showing, and therefore which credits are owed.
 enum BaseMapKind {
-  /// A real vector base map from [Env.mapStyleUrl]. Carries OpenStreetMap data,
-  /// so the ODbL credit is mandatory.
-  configured,
+  /// The default: free OpenStreetMap vector tiles from OpenFreeMap. No API key,
+  /// no registration, no request limits. Owes the OpenMapTiles and
+  /// OpenStreetMap credits.
+  openFreeMap,
+
+  /// A style URL supplied through `MAP_STYLE_URL`. Assumed to be OSM-derived,
+  /// since every practical alternative is, so it owes the same credits.
+  custom,
 
   /// The bundled fallback: a flat background and the region outline, drawn from
   /// the region config. Contains no third-party map data, so crediting
@@ -16,23 +21,33 @@ enum BaseMapKind {
 }
 
 /// Resolves the MapLibre style the app should load.
-///
-/// With no `MAP_STYLE_URL` configured the app must still run — offline, in
-/// tests, and on a fresh clone with no credentials — so it falls back to a
-/// style generated from the region's own bounding box.
 class MapStyle {
   const MapStyle({required this.kind, required this.styleString});
 
+  /// OpenFreeMap's Positron style.
+  ///
+  /// Chosen over Liberty and Bright because a radar overlay has to be the
+  /// loudest thing on screen. Positron is a desaturated grey base designed for
+  /// exactly this — the precipitation colours read against it instead of
+  /// competing with road casings and landuse fills.
+  ///
+  /// Free, unmetered and key-less: <https://openfreemap.org>.
+  static const String openFreeMapStyleUrl =
+      'https://tiles.openfreemap.org/styles/positron';
+
   factory MapStyle.forRegion(RegionConfig region) {
-    if (Env.hasMapStyle) {
+    if (Env.wantsOfflineStyle) {
       return MapStyle(
-        kind: BaseMapKind.configured,
-        styleString: Env.mapStyleUrl,
+        kind: BaseMapKind.offlineFallback,
+        styleString: buildFallbackStyle(region),
       );
     }
-    return MapStyle(
-      kind: BaseMapKind.offlineFallback,
-      styleString: buildFallbackStyle(region),
+    if (Env.hasCustomMapStyle) {
+      return MapStyle(kind: BaseMapKind.custom, styleString: Env.mapStyleUrl);
+    }
+    return const MapStyle(
+      kind: BaseMapKind.openFreeMap,
+      styleString: openFreeMapStyleUrl,
     );
   }
 
@@ -43,9 +58,14 @@ class MapStyle {
   final String styleString;
 
   /// Attribution ids owed by the base map itself, as they appear in the region
-  /// config. The radar and forecast layers add their own on top of these.
+  /// config. The radar layer adds its own on top of these.
+  ///
+  /// OpenFreeMap's own credit is optional by their terms, so it lives on the
+  /// Sources screen rather than in the always-visible bar; the OpenMapTiles and
+  /// OpenStreetMap credits are mandatory and go in the bar.
   Set<String> get attributionIds => switch (kind) {
-    BaseMapKind.configured => const <String>{'osm'},
+    BaseMapKind.openFreeMap ||
+    BaseMapKind.custom => const <String>{'openmaptiles', 'osm'},
     BaseMapKind.offlineFallback => const <String>{},
   };