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/l10n/app_localizations_zh.dart

@@ -53,6 +53,60 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get removeDocument => '移除文档';
 
+  @override
+  String get ok => '确定';
+
+  @override
+  String get pickColor => '选择颜色';
+
+  @override
+  String get clearSettingsTitle => '清除所有本地设置？';
+
+  @override
+  String get clear => '清除';
+
+  @override
+  String get settingsReset => '设置已恢复默认';
+
+  @override
+  String get themeSystem => '跟随系统';
+
+  @override
+  String get themeLight => '浅色';
+
+  @override
+  String get themeDark => '深色';
+
+  @override
+  String get seedColorDesc => 'Material 3 主题种子色';
+
+  @override
+  String get searchHint => '搜索笔记和文档…';
+
+  @override
+  String searchError(String error) {
+    return '搜索出错：$error';
+  }
+
+  @override
+  String noResultsFor(String query) {
+    return '没有“$query”的结果';
+  }
+
+  @override
+  String get typeToSearch => '输入以搜索你的笔记和文档';
+
+  @override
+  String get sectionNotes => '笔记';
+
+  @override
+  String get sectionDocuments => '文档';
+
+  @override
+  String pageLabel(int page) {
+    return '第 $page 页';
+  }
+
   @override
   String get processingPptx => '正在处理 PPTX…';