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"

tests/unit/string_ext/test_string_ext.c

@@ -214,6 +214,150 @@ static void test_clear_string_list(void **state)
     assert_null(list.items);
 }
 
+/* ==================== string_list_insert_at 测试 ==================== */
+
+static void test_insert_at_beginning(void **state)
+{
+    (void)state;
+    StringList list;
+    init_string_list(&list);
+
+    add_string_list(&list, "B");
+    add_string_list(&list, "C");
+
+    int result = string_list_insert_at(&list, 0, "A");
+
+    assert_int_equal(result, 0);
+    assert_int_equal(list.count, 3);
+    assert_string_equal(string_list_get(&list, 0), "A");
+    assert_string_equal(string_list_get(&list, 1), "B");
+    assert_string_equal(string_list_get(&list, 2), "C");
+
+    clear_string_list(&list);
+}
+
+static void test_insert_at_middle(void **state)
+{
+    (void)state;
+    StringList list;
+    init_string_list(&list);
+
+    add_string_list(&list, "A");
+    add_string_list(&list, "C");
+
+    int result = string_list_insert_at(&list, 1, "B");
+
+    assert_int_equal(result, 0);
+    assert_int_equal(list.count, 3);
+    assert_string_equal(string_list_get(&list, 0), "A");
+    assert_string_equal(string_list_get(&list, 1), "B");
+    assert_string_equal(string_list_get(&list, 2), "C");
+
+    clear_string_list(&list);
+}
+
+static void test_insert_at_end(void **state)
+{
+    (void)state;
+    StringList list;
+    init_string_list(&list);
+
+    add_string_list(&list, "A");
+
+    int result = string_list_insert_at(&list, 1, "B");
+
+    assert_int_equal(result, 0);
+    assert_int_equal(list.count, 2);
+    assert_string_equal(string_list_get(&list, 0), "A");
+    assert_string_equal(string_list_get(&list, 1), "B");
+
+    clear_string_list(&list);
+}
+
+static void test_insert_at_empty_list(void **state)
+{
+    (void)state;
+    StringList list;
+    init_string_list(&list);
+
+    int result = string_list_insert_at(&list, 0, "A");
+
+    assert_int_equal(result, 0);
+    assert_int_equal(list.count, 1);
+    assert_string_equal(string_list_get(&list, 0), "A");
+
+    clear_string_list(&list);
+}
+
+static void test_insert_at_invalid_index(void **state)
+{
+    (void)state;
+    StringList list;
+    init_string_list(&list);
+
+    add_string_list(&list, "A");
+
+    assert_int_equal(string_list_insert_at(&list, -1, "B"), -1);
+    assert_int_equal(string_list_insert_at(&list, 5, "B"), -1);
+    assert_int_equal(list.count, 1);
+
+    clear_string_list(&list);
+}
+
+static void test_insert_at_null(void **state)
+{
+    (void)state;
+    assert_int_equal(string_list_insert_at(NULL, 0, "A"), -1);
+
+    StringList list;
+    init_string_list(&list);
+    assert_int_equal(string_list_insert_at(&list, 0, NULL), -1);
+
+    clear_string_list(&list);
+}
+
+/* ==================== string_list_contains 测试 ==================== */
+
+static void test_contains_found(void **state)
+{
+    (void)state;
+    StringList list;
+    init_string_list(&list);
+
+    add_string_list(&list, "C:\\Windows");
+    add_string_list(&list, "C:\\Program Files");
+
+    assert_int_equal(string_list_contains(&list, "C:\\Windows"), 1);
+    assert_int_equal(string_list_contains(&list, "c:\\windows"), 1);  /* 不区分大小写 */
+
+    clear_string_list(&list);
+}
+
+static void test_contains_not_found(void **state)
+{
+    (void)state;
+    StringList list;
+    init_string_list(&list);
+
+    add_string_list(&list, "C:\\Windows");
+
+    assert_int_equal(string_list_contains(&list, "D:\\Tools"), 0);
+
+    clear_string_list(&list);
+}
+
+static void test_contains_null(void **state)
+{
+    (void)state;
+    assert_int_equal(string_list_contains(NULL, "test"), 0);
+
+    StringList list;
+    init_string_list(&list);
+    assert_int_equal(string_list_contains(&list, NULL), 0);
+
+    clear_string_list(&list);
+}
+
 /* ==================== 编码转换测试 ==================== */
 
 static void test_utf8_to_wide_normal(void **state)
@@ -335,6 +479,19 @@ int main(void)
         cmocka_unit_test(test_string_list_set_null_list),
         cmocka_unit_test(test_clear_string_list),
 
+        /* insert_at 测试 */
+        cmocka_unit_test(test_insert_at_beginning),
+        cmocka_unit_test(test_insert_at_middle),
+        cmocka_unit_test(test_insert_at_end),
+        cmocka_unit_test(test_insert_at_empty_list),
+        cmocka_unit_test(test_insert_at_invalid_index),
+        cmocka_unit_test(test_insert_at_null),
+
+        /* contains 测试 */
+        cmocka_unit_test(test_contains_found),
+        cmocka_unit_test(test_contains_not_found),
+        cmocka_unit_test(test_contains_null),
+
         /* 编码转换测试 */
         cmocka_unit_test(test_utf8_to_wide_normal),
         cmocka_unit_test(test_utf8_to_wide_null),