feat(programs): Excel 模板下载 + .xlsx 解析导入

- 新增 excel 4.0.6 / path_provider 2.1.5 依赖
- ExcelTemplateService：生成 Programs + Steps 双表模板（保存到应用文档目录）
- ExcelImportService：解析 .xlsx 并写入数据库，跳过已存在 code、按 program_code 关联步骤
- programs_page 顶部新增「下载模板」按钮，导入按钮改用 Excel 解析
- 移除被取代的 program_import_service.dart
- AppLocalizations 新增 downloadTemplate 键
- 验证：flutter analyze 无新增 issue；flutter build apk --debug 通过

lib/core/localization/app_localizations.dart

@@ -30,6 +30,7 @@ class AppLocalizations {
   String get editProgram => _localizedValues[locale.languageCode]?['editProgram'] ?? '编辑程序';
   String get deleteProgram => _localizedValues[locale.languageCode]?['deleteProgram'] ?? '删除程序';
   String get importProgram => _localizedValues[locale.languageCode]?['importProgram'] ?? '导入程序';
+  String get downloadTemplate => _localizedValues[locale.languageCode]?['downloadTemplate'] ?? '下载模板';
   String get viewDetails => _localizedValues[locale.languageCode]?['viewDetails'] ?? '查看详情';
   String get selectedProgram => _localizedValues[locale.languageCode]?['selectedProgram'] ?? '当前选中程序';
   String get selectedProgramLabel => _localizedValues[locale.languageCode]?['selectedProgramLabel'] ?? '当前选中';
@@ -144,6 +145,7 @@ class AppLocalizations {
       'editProgram': '编辑程序',
       'deleteProgram': '删除程序',
       'importProgram': '导入程序',
+      'downloadTemplate': '下载模板',
       'viewDetails': '查看详情',
       'selectedProgram': '当前选中程序',
       'selectedProgramLabel': '当前选中',
@@ -245,6 +247,7 @@ class AppLocalizations {
       'editProgram': 'Edit Program',
       'deleteProgram': 'Delete Program',
       'importProgram': 'Import Program',
+      'downloadTemplate': 'Download Template',
       'viewDetails': 'View Details',
       'selectedProgram': 'Selected Program',
       'selectedProgramLabel': 'Selected',