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/main.dart

@@ -0,0 +1,70 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'core/config/env.dart';
+import 'core/region/region_repository.dart';
+import 'l10n/app_localizations.dart';
+
+void main() {
+  runApp(const ProviderScope(child: NuvolariApp()));
+}
+
+class NuvolariApp extends StatelessWidget {
+  const NuvolariApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      onGenerateTitle: (context) => AppLocalizations.of(context).appTitle,
+      localizationsDelegates: AppLocalizations.localizationsDelegates,
+      supportedLocales: AppLocalizations.supportedLocales,
+      theme: ThemeData(
+        colorScheme: ColorScheme.fromSeed(seedColor: const Color(0xFF1F6FB2)),
+      ),
+      home: const HomeScreen(),
+    );
+  }
+}
+
+/// Placeholder shell for milestone 1.
+///
+/// It exists to prove the pieces are wired: localisation resolves, the region
+/// config parses out of the asset bundle, and demo mode is visible. The radar
+/// map replaces it in milestone 2.
+class HomeScreen extends ConsumerWidget {
+  const HomeScreen({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final l10n = AppLocalizations.of(context);
+    final region = ref.watch(regionConfigProvider);
+
+    return Scaffold(
+      appBar: AppBar(title: Text(l10n.appTitle)),
+      body: Column(
+        children: [
+          if (Env.isDemoMode)
+            MaterialBanner(
+              content: Text(l10n.demoModeBanner),
+              actions: const [SizedBox.shrink()],
+            ),
+          Expanded(
+            child: Center(
+              child: switch (region) {
+                AsyncData(:final value) => Text(
+                  value.displayName,
+                  style: Theme.of(context).textTheme.headlineMedium,
+                ),
+                AsyncError(:final error) => Padding(
+                  padding: const EdgeInsets.all(24),
+                  child: Text('${l10n.dataUnavailable}\n$error'),
+                ),
+                _ => Text(l10n.loading),
+              },
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}