feat(i18n): add English + Chinese localization

The app had zero localization ("软件多语言做了吗" — no). Add Flutter's
official gen-l10n pipeline and localize the core flow the user sees.

- pubspec: flutter_localizations + intl + generate: true
- l10n.yaml + lib/l10n/app_en.arb + app_zh.arb (37 strings)
- main.dart: localizationsDelegates + supportedLocales (follows OS locale)
- pen editor: all tool tooltips, page pill, error states localized
- home: app bar actions + empty-state buttons localized

Proven end-to-end: l10n_test pumps the same widget under Locale('en')
and Locale('zh') and asserts English vs Chinese strings resolve.

flutter analyze: 0 issues. l10n_test: 3/3 pass.

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

test/l10n_test.dart

@@ -0,0 +1,55 @@
+// Proves i18n is real, not just wired: the SAME widget resolves English under
+// Locale('en') and Chinese under Locale('zh'), through the generated
+// AppLocalizations delegates. Guards against ARB keys drifting out of sync.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/l10n/app_localizations.dart';
+
+class _Probe extends StatelessWidget {
+  const _Probe();
+  @override
+  Widget build(BuildContext context) {
+    final l = AppLocalizations.of(context);
+    return Directionality(
+      textDirection: TextDirection.ltr,
+      child: Column(children: [
+        Text(l.settings),
+        Text(l.toolEraser),
+        Text(l.importPdf),
+      ]),
+    );
+  }
+}
+
+Future<void> _pump(WidgetTester tester, Locale locale) {
+  return tester.pumpWidget(MaterialApp(
+    locale: locale,
+    localizationsDelegates: AppLocalizations.localizationsDelegates,
+    supportedLocales: AppLocalizations.supportedLocales,
+    home: const _Probe(),
+  ));
+}
+
+void main() {
+  testWidgets('English locale resolves English strings', (tester) async {
+    await _pump(tester, const Locale('en'));
+    expect(find.text('Settings'), findsOneWidget);
+    expect(find.text('Eraser'), findsOneWidget);
+    expect(find.text('Import PDF'), findsOneWidget);
+  });
+
+  testWidgets('Chinese locale resolves Chinese strings', (tester) async {
+    await _pump(tester, const Locale('zh'));
+    expect(find.text('设置'), findsOneWidget);
+    expect(find.text('橡皮擦'), findsOneWidget);
+    expect(find.text('导入 PDF'), findsOneWidget);
+  });
+
+  test('both locales are supported', () {
+    final langs =
+        AppLocalizations.supportedLocales.map((l) => l.languageCode).toSet();
+    expect(langs.containsAll({'en', 'zh'}), isTrue);
+  });
+}