Scaffold the Flutter app with Italian l10n and region configuration

Creates the app skeleton and the seam everything else in the project hangs
off: region-specific data lives in an asset file, not in code, so adding a
region later is a new JSON file rather than a refactor.

Notable choices:

- applicationId and namespace are it.nuvolari.app rather than the doubled
  it.nuvolari.nuvolari that `flutter create` produces, with the Kotlin package
  and the iOS bundle identifiers moved to match.
- SDK levels are pinned instead of inherited from `flutter.*`. Google Play
  requires API 36, and that is a release blocker rather than something to let a
  Flutter upgrade change silently. minSdk 24 is the highest floor the planned
  dependencies impose.
- Riverpod and Dio-free for now, no code generation: see docs/stack-decisions.md.
- Italian is the l10n source language, so app_it.arb is the template rather
  than a translation of an English original.

The region parser rejects rather than repairs. An inverted bounding box, a map
centre outside its own bounds, an unknown adapter name, a duplicate zone code
or an empty attribution list all throw with the offending field named. Each of
those would otherwise fail silently and visibly wrong: a swapped latitude and
longitude renders the radar in the wrong place, an unknown adapter falling back
to mock would show demo frames where live data was expected, and a missing
attribution is a licence violation rather than a cosmetic gap.

Tests run against the asset that actually ships and against a captured copy of
the live ARPA CAP feed, so the eleven zone codes in the config are checked
against the eleven the feed really emits rather than against a list retyped
from documentation.

Verified: dart format clean, flutter analyze 0 issues, 50 tests passing,
flutter build appbundle --debug produces an AAB with applicationId
it.nuvolari.app, minSdk 24, targetSdk 36.

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

Nuvolari/app/lib/core/config/env.dart

@@ -0,0 +1,65 @@
+/// Build-time configuration, supplied with `--dart-define-from-file=env.json`.
+///
+/// Every value has a working default, so the app builds and runs with no
+/// `env.json` at all: it starts in demo mode against bundled mock frames, with
+/// the offline map style and Google's test ad units. That keeps the repository
+/// free of secrets and keeps a fresh clone runnable.
+///
+/// Values are `const` reads of `String.fromEnvironment` so they are inlined and
+/// tree-shaken; they cannot be read from a file at run time.
+library;
+
+class Env {
+  const Env._();
+
+  /// Which region config to load from `assets/regions/<id>.json`.
+  static const String regionId = String.fromEnvironment(
+    'REGION_ID',
+    defaultValue: 'piemonte',
+  );
+
+  /// MapLibre style URL for the base map.
+  ///
+  /// Empty means no key is configured, and [hasMapStyle] is false: the app
+  /// falls back to a minimal style bundled in assets so development and tests
+  /// work offline.
+  static const String mapStyleUrl = String.fromEnvironment('MAP_STYLE_URL');
+
+  static bool get hasMapStyle => mapStyleUrl.isNotEmpty;
+
+  /// Base URL of the radar `manifest.json` published by our worker.
+  ///
+  /// Empty forces the mock radar source regardless of [radarSource].
+  static const String radarManifestUrl = String.fromEnvironment(
+    'RADAR_MANIFEST_URL',
+  );
+
+  /// Radar adapter override: `mock`, `dpc` or `arpa`.
+  ///
+  /// Empty means "use whatever the region config says".
+  static const String radarSource = String.fromEnvironment('RADAR_SOURCE');
+
+  /// Contact address embedded in the MET Norway User-Agent.
+  ///
+  /// MET Norway blocks generic User-Agent strings, so forecasts are disabled
+  /// rather than attempted when this is empty — see [hasForecastContact].
+  static const String metnoUserAgentContact = String.fromEnvironment(
+    'METNO_USER_AGENT_CONTACT',
+  );
+
+  static bool get hasForecastContact => metnoUserAgentContact.isNotEmpty;
+
+  /// AdMob identifiers. Empty means Google's test units are used.
+  static const String admobAppId = String.fromEnvironment('ADMOB_APP_ID');
+  static const String admobBannerUnitId = String.fromEnvironment(
+    'ADMOB_BANNER_UNIT_ID',
+  );
+
+  static bool get hasAdMobConfig =>
+      admobAppId.isNotEmpty && admobBannerUnitId.isNotEmpty;
+
+  /// True when nothing is configured and the app is running entirely on
+  /// bundled data. Surfaced in the UI so demo frames are never mistaken for a
+  /// real forecast.
+  static bool get isDemoMode => radarManifestUrl.isEmpty;
+}