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/editor/canvas/pen_editor_screen.dart

@@ -8,6 +8,7 @@
 import 'package:flutter/material.dart';
 import 'package:pdfrx/pdfrx.dart';
 
+import '../../l10n/app_localizations.dart';
 import '../../services/database_service.dart';
 import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
 import '../engine/stroke_model.dart';
@@ -398,6 +399,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
 
   @override
   Widget build(BuildContext context) {
+    final l = AppLocalizations.of(context);
     return Scaffold(
       body: Stack(
         children: [
@@ -429,7 +431,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
               padding: const EdgeInsets.all(8),
               child: _RoundIconButton(
                 icon: Icons.arrow_back,
-                tooltip: 'Back',
+                tooltip: l.back,
                 onPressed: () => Navigator.of(context).maybePop(),
               ),
             ),
@@ -510,15 +512,16 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   }
 
   Widget _buildBody() {
+    final l = AppLocalizations.of(context);
     if (_openError != null) {
-      return Center(child: Text('Failed to open PDF:\n$_openError'));
+      return Center(child: Text(l.failedToOpenPdf('$_openError')));
     }
     final doc = _document;
     if (doc == null) {
       return const Center(child: CircularProgressIndicator());
     }
     if (doc.pages.isEmpty) {
-      return const Center(child: Text('PDF has no pages.'));
+      return Center(child: Text(l.pdfNoPages));
     }
 
     final page = doc.pages[_pageIndex];
@@ -584,6 +587,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   /// tools, color dots, and finger-drawing toggle.
   Widget _buildToolPalette() {
     final cs = Theme.of(context).colorScheme;
+    final l = AppLocalizations.of(context);
     return Material(
       color: cs.surfaceContainerHigh,
       elevation: 3,
@@ -596,19 +600,19 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
             _ToolButton(
               icon: Icons.edit_outlined,
               selected: _tool == CanvasTool.pen,
-              tooltip: 'Pen',
+              tooltip: l.toolPen,
               onPressed: () => setState(() => _tool = CanvasTool.pen),
             ),
             _ToolButton(
               icon: Icons.brush_outlined,
               selected: _tool == CanvasTool.highlighter,
-              tooltip: 'Highlighter',
+              tooltip: l.toolHighlighter,
               onPressed: () => setState(() => _tool = CanvasTool.highlighter),
             ),
             _ToolButton(
               icon: Icons.cleaning_services_outlined,
               selected: _tool == CanvasTool.eraser,
-              tooltip: 'Eraser',
+              tooltip: l.toolEraser,
               onPressed: () => setState(() => _tool = CanvasTool.eraser),
             ),
             _Divider(cs: cs),
@@ -616,13 +620,13 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
             _ToolButton(
               icon: Icons.undo,
               selected: false,
-              tooltip: 'Undo',
+              tooltip: l.actionUndo,
               onPressed: _undoFor(_pageIndex).canUndo ? _performUndo : null,
             ),
             _ToolButton(
               icon: Icons.redo,
               selected: false,
-              tooltip: 'Redo',
+              tooltip: l.actionRedo,
               onPressed: _undoFor(_pageIndex).canRedo ? _performRedo : null,
             ),
             _Divider(cs: cs),
@@ -632,28 +636,28 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
               icon: _allowFingerDrawing ? Icons.touch_app : Icons.do_not_touch,
               selected: _allowFingerDrawing,
               tooltip: _allowFingerDrawing
-                  ? 'Finger drawing ON'
-                  : 'Finger drawing OFF (pen only)',
+                  ? l.fingerDrawingOn
+                  : l.fingerDrawingOff,
               onPressed: _toggleFingerDrawing,
             ),
             // Page thumbnail grid.
             _ToolButton(
               icon: Icons.grid_view,
               selected: false,
-              tooltip: 'Pages',
+              tooltip: l.pages,
               onPressed: _document != null ? _openThumbnails : null,
             ),
             // Pen settings.
             _ToolButton(
               icon: Icons.settings_outlined,
               selected: false,
-              tooltip: 'Pen settings',
+              tooltip: l.penSettings,
               onPressed: _penConfig != null ? _openPenSettings : null,
             ),
             _ToolButton(
               icon: Icons.bug_report_outlined,
               selected: _showPenDebug,
-              tooltip: 'Input diagnostic (writes a log file)',
+              tooltip: l.inputDiagnostic,
               onPressed: () {
                 final on = !_showPenDebug;
                 setState(() => _showPenDebug = on);
@@ -699,6 +703,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   Widget _buildPagePill() {
     final doc = _document!;
     final cs = Theme.of(context).colorScheme;
+    final l = AppLocalizations.of(context);
     final total = doc.pages.length;
     final shown = (_scrub ?? (_pageIndex + 1).toDouble()).round();
     return Column(
@@ -742,7 +747,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
               mainAxisSize: MainAxisSize.min,
               children: [
                 IconButton(
-                  tooltip: 'Previous page',
+                  tooltip: l.previousPage,
                   icon: const Icon(Icons.chevron_left),
                   onPressed:
                       _pageIndex > 0 ? () => _goToPage(_pageIndex - 1) : null,
@@ -752,7 +757,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
                       ? () => setState(() => _showSlider = !_showSlider)
                       : null,
                   child: Text(
-                    '$shown / $total',
+                    l.pageOfPages(shown, total),
                     style: TextStyle(
                       color: cs.onSurface,
                       fontWeight: FontWeight.w600,
@@ -760,7 +765,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
                   ),
                 ),
                 IconButton(
-                  tooltip: 'Next page',
+                  tooltip: l.nextPage,
                   icon: const Icon(Icons.chevron_right),
                   onPressed: _pageIndex < total - 1
                       ? () => _goToPage(_pageIndex + 1)