注解 @Injectable 的作用：
Determines which injectors will provide the injectable, by either associating it with an @NgModule or other InjectorType, or by specifying that this injectable should be provided in one of the following injectors:
‘root’ : The application-level injector in most apps.
‘platform’ : A special singleton platform injector shared by all applications on the page.
‘any’ : Provides a unique instance in each lazy loaded module while all eagerly loaded modules share one instance.
Marking a class with @Injectable ensures that the compiler will generate the necessary metadata to create the class’s dependencies when the class is injected.
@Injectable 的含義是，當被注解的 class 被注入時，編譯器會生成必要的中繼資料，以建立類的依賴。