feat: CSV 导入导出、导入撤销支持及多项 bug 修复

功能：
- 新增 CSV 格式导入导出支持（含 BOM 处理、引号转义、智能标题行检测）
- 导入操作支持撤销/重做
- 保存时 PATH 长度检查与警告
- 深色模式状态持久化（darkmode.txt）
- 提取 get_current_target/push_record 为共享函数，消除控制器层重复代码
- 新增 string_list_insert_at，修复撤销删除时的索引恢复
- 新增 undo_redo、error_code、import_export 单元测试

Bug 修复：
- 修复备份目录对话框和失败原因的硬编码中文字符串
- 提取 get_exe_dir 到 os_env 消除 i18n.c/ui_utils.c 重复定义
- 修复导入撤销 old_sys/old_user 内存管理（push 后置 NULL 防止重复释放）
- 修复 CSV 导出转义与导入解析不一致（移除反斜杠转义，依赖 CSV 引号机制）
- 修正 PATH 长度 8191 限制描述为 "command line safe limit"

po/en_US.po

@@ -414,4 +414,32 @@ msgstr "Dark Mode"
 
 #: src/ui/main_window.c
 msgid "Light Mode"
-msgstr "Light Mode"
+msgstr "Light Mode"
+
+#: src/controller/callbacks_sys.c
+msgid "Select backup directory"
+msgstr "Select backup directory"
+
+#: src/controller/callbacks_sys.c
+msgid "Unknown error"
+msgstr "Unknown error"
+
+#: src/controller/callbacks_sys.c
+msgid "Failed to get AppData path"
+msgstr "Failed to get AppData path"
+
+#: src/controller/callbacks_sys.c
+msgid "Failed to create backup directory or file"
+msgstr "Failed to create backup directory or file"
+
+#: src/controller/callbacks_sys.c
+msgid "Failed to read PATH from registry"
+msgstr "Failed to read PATH from registry"
+
+#: src/controller/callbacks_sys.c
+msgid "Backup failed! Reason: %s\n\nContinue saving?\n(Continuing may prevent recovery)"
+msgstr "Backup failed! Reason: %s\n\nContinue saving?\n(Continuing may prevent recovery)"
+
+#: src/controller/callbacks_sys.c
+msgid "Total PATH length: %d characters (command line safe limit: 8191)\n"
+msgstr "Total PATH length: %d characters (command line safe limit: 8191)\n"