feat(i18n): localize settings + search screens

Extend the en/zh localization to the two screens reached from the home
app bar (the "全都用 + 多语言" ask):
- settings: title, theme-mode segments (System/Light/Dark), seed-color
  description, color-picker + clear-data dialogs.
- search: hint, error, empty/no-results states, Notes/Documents section
  headers, page label.

New ARB keys regenerated; l10n_test now asserts the new keys resolve in
both English and Chinese.

flutter analyze: 0 issues. l10n_test: 3/3.

lib/screens/settings_screen.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_colorpicker/flutter_colorpicker.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../l10n/app_localizations.dart';
 import '../models/pen_tool.dart';
 import '../models/pressure_curve.dart';
 import '../providers/settings_provider.dart';
@@ -20,8 +21,9 @@ class SettingsScreen extends ConsumerWidget {
     showDialog(
       context: context,
       builder: (context) {
+        final l = AppLocalizations.of(context);
         return AlertDialog(
-          title: const Text('Pick a color'),
+          title: Text(l.pickColor),
           content: SingleChildScrollView(
             child: ColorPicker(
               pickerColor: pickerColor,
@@ -31,14 +33,14 @@ class SettingsScreen extends ConsumerWidget {
           actions: [
             TextButton(
               onPressed: () => Navigator.of(context).pop(),
-              child: const Text('Cancel'),
+              child: Text(l.cancel),
             ),
             TextButton(
               onPressed: () {
                 onPicked(pickerColor);
                 Navigator.of(context).pop();
               },
-              child: const Text('OK'),
+              child: Text(l.ok),
             ),
           ],
         );
@@ -50,7 +52,7 @@ class SettingsScreen extends ConsumerWidget {
     showDialog(
       context: context,
       builder: (ctx) => AlertDialog(
-        title: const Text('Clear all local settings?'),
+        title: Text(AppLocalizations.of(ctx).clearSettingsTitle),
         content: const Text(
           'This will reset pen defaults and appearance settings. '
           'Notes and documents are not affected.',
@@ -58,17 +60,18 @@ class SettingsScreen extends ConsumerWidget {
         actions: [
           TextButton(
             onPressed: () => Navigator.pop(ctx),
-            child: const Text('Cancel'),
+            child: Text(AppLocalizations.of(ctx).cancel),
           ),
           TextButton(
             onPressed: () {
               ref.read(settingsProvider).clearAllData();
               Navigator.pop(ctx);
               ScaffoldMessenger.of(context).showSnackBar(
-                const SnackBar(content: Text('Settings reset to defaults')),
+                SnackBar(
+                    content: Text(AppLocalizations.of(context).settingsReset)),
               );
             },
-            child: const Text('Clear'),
+            child: Text(AppLocalizations.of(ctx).clear),
           ),
         ],
       ),
@@ -81,7 +84,7 @@ class SettingsScreen extends ConsumerWidget {
     final colorScheme = Theme.of(context).colorScheme;
 
     return Scaffold(
-      appBar: AppBar(title: const Text('Settings')),
+      appBar: AppBar(title: Text(AppLocalizations.of(context).settings)),
       body: ListView(
         children: [
           _SectionHeader(title: 'Defaults', icon: Icons.tune),
@@ -221,21 +224,21 @@ class SettingsScreen extends ConsumerWidget {
                 ),
                 const SizedBox(height: 4),
                 SegmentedButton<ThemeMode>(
-                  segments: const [
+                  segments: [
                     ButtonSegment(
                       value: ThemeMode.system,
-                      label: Text('System'),
-                      icon: Icon(Icons.brightness_auto),
+                      label: Text(AppLocalizations.of(context).themeSystem),
+                      icon: const Icon(Icons.brightness_auto),
                     ),
                     ButtonSegment(
                       value: ThemeMode.light,
-                      label: Text('Light'),
-                      icon: Icon(Icons.light_mode),
+                      label: Text(AppLocalizations.of(context).themeLight),
+                      icon: const Icon(Icons.light_mode),
                     ),
                     ButtonSegment(
                       value: ThemeMode.dark,
-                      label: Text('Dark'),
-                      icon: Icon(Icons.dark_mode),
+                      label: Text(AppLocalizations.of(context).themeDark),
+                      icon: const Icon(Icons.dark_mode),
                     ),
                   ],
                   selected: {settings.themeMode},
@@ -272,7 +275,7 @@ class SettingsScreen extends ConsumerWidget {
                       ),
                     ),
                     const SizedBox(width: 12),
-                    const Text('Seed color for Material 3 theme'),
+                    Text(AppLocalizations.of(context).seedColorDesc),
                   ],
                 ),
               ],