test(i18n): cover the i18n facade, selector, manifest routing + locale (T-462)

Tests for the epic's new code so the 95% floor holds: ClideSettings.i18n
string/interpolated null-safe + localizedCommandTitle; settings-renderer
localization (section/field/help/select/file, project-scope tag, didUpdateWidget);
extension-activation rollback + settings-contribution unregister; the Dutch
gate check; RootShell applying app.locale on boot; and prompt/conversation
tool-body + file-ref coverage. Also harden the menu toggle re-tap (capture the
button point while stable, then tapAt) so it no longer throws getCenter under
the serial coverage load.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/app_test.dart

@@ -453,4 +453,21 @@ void main() {
     await tester.pump();
     expect(f.services.project.isOpen, isFalse);
   });
+
+  testWidgets('RootShell applies the persisted app.locale on boot (T-462)', (tester) async {
+    await tester.runAsync(() async => f.services.settings.set('app.locale', 'nl_NL'));
+    await pumpApp(tester);
+    expect(tester.takeException(), isNull);
+    // RootShell.initState parsed app.locale and called i18n.setLocale.
+    expect(f.services.i18n.currentLocale, const Locale('nl', 'NL'));
+  });
+
+  testWidgets('RootShell applies a bare language-only app.locale on boot (T-462)', (tester) async {
+    // A single-part locale ('nl', no region) exercises the Locale(parts[0])
+    // branch in _applyLocale, distinct from the language_REGION form above.
+    await tester.runAsync(() async => f.services.settings.set('app.locale', 'nl'));
+    await pumpApp(tester);
+    expect(tester.takeException(), isNull);
+    expect(f.services.i18n.currentLocale, const Locale('nl'));
+  });
 }