Change subtitle heading to new slogan

- Unify reading subtitle/slogan throughout the application.
- Refactor related unit tests for easier future changes.
- Add typed constants for Vue app environment variables.

src/presentation/electron/Update/ManualUpdater.ts

@@ -32,6 +32,7 @@ function getTargetProject(targetVersion: string) {
   const targetProject = new ProjectInformation(
     existingProject.name,
     new Version(targetVersion),
+    existingProject.slogan,
     existingProject.repositoryUrl,
     existingProject.homepage,
   );