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>

lib/l10n/app_localizations_zh.dart

@@ -0,0 +1,116 @@
+// ignore: unused_import
+import 'package:intl/intl.dart' as intl;
+import 'app_localizations.dart';
+
+// ignore_for_file: type=lint
+
+/// The translations for Chinese (`zh`).
+class AppLocalizationsZh extends AppLocalizations {
+  AppLocalizationsZh([String locale = 'zh']) : super(locale);
+
+  @override
+  String get appTitle => 'BadNote';
+
+  @override
+  String get settings => '设置';
+
+  @override
+  String get search => '搜索';
+
+  @override
+  String get importPdf => '导入 PDF';
+
+  @override
+  String get importPpt => '导入 PPT';
+
+  @override
+  String get penCanvasBeta => '手写画布（测试版）';
+
+  @override
+  String get newNote => '新建笔记';
+
+  @override
+  String get open => '打开';
+
+  @override
+  String get cancel => '取消';
+
+  @override
+  String get delete => '删除';
+
+  @override
+  String get deleteNoteTitle => '删除笔记？';
+
+  @override
+  String get deleteNote => '删除笔记';
+
+  @override
+  String get openInSplitView => '分屏打开';
+
+  @override
+  String get splitViewSubtitle => 'PDF 参考 + 草稿纸';
+
+  @override
+  String get removeDocument => '移除文档';
+
+  @override
+  String get processingPptx => '正在处理 PPTX…';
+
+  @override
+  String get processingPresentation => '正在处理演示文稿…';
+
+  @override
+  String get couldNotOpenPresentation => '无法打开演示文稿。';
+
+  @override
+  String get toolPen => '钢笔';
+
+  @override
+  String get toolHighlighter => '荧光笔';
+
+  @override
+  String get toolEraser => '橡皮擦';
+
+  @override
+  String get actionUndo => '撤销';
+
+  @override
+  String get actionRedo => '重做';
+
+  @override
+  String get fingerDrawingOn => '手指书写：开';
+
+  @override
+  String get fingerDrawingOff => '手指书写：关（仅手写笔）';
+
+  @override
+  String get pages => '页面';
+
+  @override
+  String get penSettings => '手写笔设置';
+
+  @override
+  String get inputDiagnostic => '输入诊断（写入日志文件）';
+
+  @override
+  String get back => '返回';
+
+  @override
+  String get previousPage => '上一页';
+
+  @override
+  String get nextPage => '下一页';
+
+  @override
+  String failedToOpenPdf(String error) {
+    return '打开 PDF 失败：\n$error';
+  }
+
+  @override
+  String get pdfNoPages => 'PDF 没有任何页面。';
+
+  @override
+  String pageOfPages(int current, int total) {
+    return '$current / $total';
+  }
+}