fix: 设为纯托盘应用，启动不再在Dock显示图标

将Info.plist的LSUIElement改为true，应用以accessory模式启动，
永不显示Dock图标和系统菜单栏。移除运行时切换Dock可见性的
showDockIcon()/hideDockIcon()及其C实现setDockIconVisible，
这些调用在accessory模式下已无意义。activateApp()保留用于
窗口置于前台获取焦点。

internal/ui/dock_darwin.go

@@ -25,12 +25,6 @@ static BOOL appShouldHandleReopen(id self, SEL cmd, id sender, BOOL flag) {
     return YES;
 }
 
-static void setDockIconVisible(int visible) {
-	Class cls = objc_getClass("NSApplication");
-	id app = ((id (*)(Class, SEL))objc_msgSend)(cls, sel_getUid("sharedApplication"));
-	((void (*)(id, SEL, long))objc_msgSend)(app, sel_getUid("setActivationPolicy:"), visible ? 0 : 1);
-}
-
 static void cmActivateApp(void) {
 	Class cls = objc_getClass("NSApplication");
 	id app = ((id (*)(Class, SEL))objc_msgSend)(cls, sel_getUid("sharedApplication"));
@@ -53,9 +47,6 @@ static void cmRegisterReopenHandler(void) {
 */
 import "C"
 
-func showDockIcon() { C.setDockIconVisible(1) }
-func hideDockIcon() { C.setDockIconVisible(0) }
-
 func activateApp() { C.cmActivateApp() }
 
 func registerReopenHandler() { C.cmRegisterReopenHandler() }