fix(auth): add better signin signup logic

src/app/header/header.component.html

src/app/header/header.component.ts

src/app/models/user-auth.model.ts

src/app/services/auth.service.ts

...hared/components/signin-modal/signin-modal.component.html

...hared/components/signup-modal/signup-modal.component.html

.../shared/components/signup-modal/signup-modal.component.ts