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_en.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 English (`en`).
+class AppLocalizationsEn extends AppLocalizations {
+  AppLocalizationsEn([String locale = 'en']) : super(locale);
+
+  @override
+  String get appTitle => 'BadNote';
+
+  @override
+  String get settings => 'Settings';
+
+  @override
+  String get search => 'Search';
+
+  @override
+  String get importPdf => 'Import PDF';
+
+  @override
+  String get importPpt => 'Import PPT';
+
+  @override
+  String get penCanvasBeta => 'Pen Canvas (beta)';
+
+  @override
+  String get newNote => 'New Note';
+
+  @override
+  String get open => 'Open';
+
+  @override
+  String get cancel => 'Cancel';
+
+  @override
+  String get delete => 'Delete';
+
+  @override
+  String get deleteNoteTitle => 'Delete note?';
+
+  @override
+  String get deleteNote => 'Delete note';
+
+  @override
+  String get openInSplitView => 'Open in Split View';
+
+  @override
+  String get splitViewSubtitle => 'PDF reference + scratchpad';
+
+  @override
+  String get removeDocument => 'Remove document';
+
+  @override
+  String get processingPptx => 'Processing PPTX...';
+
+  @override
+  String get processingPresentation => 'Processing presentation...';
+
+  @override
+  String get couldNotOpenPresentation => 'Could not open presentation.';
+
+  @override
+  String get toolPen => 'Pen';
+
+  @override
+  String get toolHighlighter => 'Highlighter';
+
+  @override
+  String get toolEraser => 'Eraser';
+
+  @override
+  String get actionUndo => 'Undo';
+
+  @override
+  String get actionRedo => 'Redo';
+
+  @override
+  String get fingerDrawingOn => 'Finger drawing ON';
+
+  @override
+  String get fingerDrawingOff => 'Finger drawing OFF (pen only)';
+
+  @override
+  String get pages => 'Pages';
+
+  @override
+  String get penSettings => 'Pen settings';
+
+  @override
+  String get inputDiagnostic => 'Input diagnostic (writes a log file)';
+
+  @override
+  String get back => 'Back';
+
+  @override
+  String get previousPage => 'Previous page';
+
+  @override
+  String get nextPage => 'Next page';
+
+  @override
+  String failedToOpenPdf(String error) {
+    return 'Failed to open PDF:\n$error';
+  }
+
+  @override
+  String get pdfNoPages => 'PDF has no pages.';
+
+  @override
+  String pageOfPages(int current, int total) {
+    return '$current / $total';
+  }
+}