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/screens/home_screen.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../l10n/app_localizations.dart';
 import '../models/document.dart';
 import '../models/note.dart';
 import '../providers/document_provider.dart';
@@ -34,15 +35,16 @@ class HomeScreen extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final notesAsync = ref.watch(noteListProvider);
+    final l = AppLocalizations.of(context);
 
     return Scaffold(
       appBar: AppBar(
-        title: const Text('BadNote'),
+        title: Text(l.appTitle),
         centerTitle: true,
         actions: [
           IconButton(
             icon: const Icon(Icons.settings),
-            tooltip: 'Settings',
+            tooltip: l.settings,
             onPressed: () {
               Navigator.of(
                 context,
@@ -51,17 +53,17 @@ class HomeScreen extends ConsumerWidget {
           ),
           IconButton(
             icon: const Icon(Icons.picture_as_pdf),
-            tooltip: 'Import PDF',
+            tooltip: l.importPdf,
             onPressed: () => _importPdf(context),
           ),
           IconButton(
             icon: const Icon(Icons.slideshow),
-            tooltip: 'Import PPT',
+            tooltip: l.importPpt,
             onPressed: () => _importPptx(context),
           ),
           IconButton(
             icon: const Icon(Icons.search),
-            tooltip: 'Search',
+            tooltip: l.search,
             onPressed: () {
               Navigator.of(
                 context,
@@ -71,7 +73,7 @@ class HomeScreen extends ConsumerWidget {
           // New pen-first canvas editor (beta).
           IconButton(
             icon: const Icon(Icons.draw_outlined),
-            tooltip: 'Pen Canvas (beta)',
+            tooltip: l.penCanvasBeta,
             onPressed: () => openM1Spike(context),
           ),
         ],
@@ -265,19 +267,19 @@ class HomeScreen extends ConsumerWidget {
           FilledButton.icon(
             onPressed: () => _createAndOpenNote(context, ref),
             icon: const Icon(Icons.add),
-            label: const Text('New Note'),
+            label: Text(AppLocalizations.of(context).newNote),
           ),
           const SizedBox(height: 12),
           OutlinedButton.icon(
             onPressed: () => _importPdf(context),
             icon: const Icon(Icons.picture_as_pdf),
-            label: const Text('Import PDF'),
+            label: Text(AppLocalizations.of(context).importPdf),
           ),
           const SizedBox(height: 12),
           OutlinedButton.icon(
             onPressed: () => _importPptx(context),
             icon: const Icon(Icons.slideshow),
-            label: const Text('Import PPT'),
+            label: Text(AppLocalizations.of(context).importPpt),
           ),
         ],
       ),