halkyon_app: data locations as precise as a country or a state

An operator's DATA_LOCATION and BACKUP_LOCATION may name a subdivision
(ISO 3166-2, US-CA, DE-BY) besides the EU or a country. The app adds
"(EU)" to a member country and the country to a subdivision ("Germany
(EU)", "California, United States", "Bavaria, Germany (EU)"), in
English and German, and shows the most specific flag there is: a
subdivision's own where the flag package has one, else its country's.
Subdivision names come from Unicode CLDR 48.2 (tool/subdivision_names.py),
bundled with their licence, which the app registers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/test/instance/widget/instance_flag_test.dart

@@ -0,0 +1,37 @@
+import 'package:flag/flag.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:material_ui/material_ui.dart';
+
+void main() {
+  test("a place shows the most specific flag there is: a subdivision's own, "
+      "else its country's, and the European Union's", () {
+    expect(flagCodeOf('EU'), 'eu');
+    expect(flagCodeOf('DE'), 'de');
+    expect(flagCodeOf('CH'), 'ch');
+    expect(flagCodeOf('DE-BY'), 'de');
+    expect(flagCodeOf('US-CA'), 'us');
+    expect(flagCodeOf('GB-SCT'), 'gb_sct');
+    expect(flagCodeOf('ES-CT'), 'es_ct');
+    expect(flagCodeOf('DE-ZZ'), 'de');
+  });
+
+  test('a place without a flag shows none', () {
+    expect(flagCodeOf('QQ'), isNull);
+    expect(flagCodeOf('QQ-AB'), isNull);
+  });
+
+  testWidgets("a state shows its country's flag, a place without one "
+      'nothing', (tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: Row(
+          children: [InstanceFlag('US-CA'), InstanceFlag('QQ-AB')],
+        ),
+      ),
+    );
+
+    expect(find.byType(Flag), findsOne);
+  });
+}