Tauri-demo: refactor the login method

auth.ts

    async #isUserRegistered(email: string) {
        const signInMethods = await fetchSignInMethodsForEmail(getAuth(), email);
        return signInMethods.length > 0;
    }

    async #registerUser(email: string, password: string) {
        await createUserWithEmailAndPassword(getAuth(), email, password);
        await sendEmailVerification(getAuth().currentUser!);
    }

    async login(email: string, password: string) {        
        if (await this.#isUserRegistered(email) === false) {
            await this.#registerUser(email, password);
        } else {
            await signInWithEmailAndPassword(getAuth(), email, password);
        }
    }