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/main.dart

@@ -1,11 +1,13 @@
 import 'package:dynamic_color/dynamic_color.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:google_fonts/google_fonts.dart';
 import 'package:pdfrx/pdfrx.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import 'editor/pdf/pen_capture_region.dart';
+import 'l10n/app_localizations.dart';
 import 'providers/settings_provider.dart';
 import 'screens/home_screen.dart';
 import 'services/database_service.dart';
@@ -54,6 +56,14 @@ class BadNoteApp extends ConsumerWidget {
           themeMode: settings.themeMode,
           theme: _theme(lightScheme),
           darkTheme: _theme(darkScheme),
+          // i18n: follows the OS language (en / zh) via the system locale.
+          localizationsDelegates: const [
+            AppLocalizations.delegate,
+            GlobalMaterialLocalizations.delegate,
+            GlobalWidgetsLocalizations.delegate,
+            GlobalCupertinoLocalizations.delegate,
+          ],
+          supportedLocales: AppLocalizations.supportedLocales,
           home: const HomeScreen(),
         );
       },