修复：部署目标降至 11.0，秒数/12小时制改为可配置

- 将 MACOSX_DEPLOYMENT_TARGET 由项目级 27.0 / 目标级 15.6 统一改为 11.0
  （代码所用 API 的最低门槛为 11.0），使屏保可在当前及更早系统加载
- showSeconds / use24Hour 由写死的 static let 改为设置驱动的可选项：
  * FlipClockSettings 新增键 showSeconds / use24Hour，支持持久化与读取
  * 设置面板新增「显示秒数」「24 小时制」两个勾选项，取消可回滚
  * 视图的 loadSettings / 跨进程轮询 / 设置通知均同步新选项
  * 原 use24Hour=false 的 AM/PM 死代码现已可经设置启用

FlipClockSaver.xcodeproj/project.pbxproj

@@ -191,7 +191,7 @@
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
 				LOCALIZATION_PREFERS_STRING_CATALOGS = YES;
-				MACOSX_DEPLOYMENT_TARGET = 27.0;
+				MACOSX_DEPLOYMENT_TARGET = 11.0;
 				MTL_ENABLE_DEBUG_INFO = INCLUDE_SOURCE;
 				MTL_FAST_MATH = YES;
 				ONLY_ACTIVE_ARCH = YES;
@@ -247,7 +247,7 @@
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
 				LOCALIZATION_PREFERS_STRING_CATALOGS = YES;
-				MACOSX_DEPLOYMENT_TARGET = 27.0;
+				MACOSX_DEPLOYMENT_TARGET = 11.0;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				MTL_FAST_MATH = YES;
 				SDKROOT = macosx;
@@ -266,7 +266,7 @@
 				INFOPLIST_KEY_NSHumanReadableCopyright = "";
 				INFOPLIST_KEY_NSPrincipalClass = FlipClockSaverView;
 				INSTALL_PATH = "$(HOME)/Library/Screen Savers";
-				MACOSX_DEPLOYMENT_TARGET = 15.6;
+				MACOSX_DEPLOYMENT_TARGET = 11.0;
 				MARKETING_VERSION = 1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = com.liucheng.FlipClockSaver;
 				PRODUCT_NAME = "$(TARGET_NAME)";
@@ -291,7 +291,7 @@
 				INFOPLIST_KEY_NSHumanReadableCopyright = "";
 				INFOPLIST_KEY_NSPrincipalClass = FlipClockSaverView;
 				INSTALL_PATH = "$(HOME)/Library/Screen Savers";
-				MACOSX_DEPLOYMENT_TARGET = 15.6;
+				MACOSX_DEPLOYMENT_TARGET = 11.0;
 				MARKETING_VERSION = 1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = com.liucheng.FlipClockSaver;
 				PRODUCT_NAME = "$(TARGET_NAME)";