Is this method of salting effective?
Sha256 has not long been cracked, but it's not useful for human-generated passwords. You want a key stretching function. SubtleCrypto provides PBKDF2 (documentation), which is fine. Other libraries might support bcrypt, scrypt, or Argon2, any of which would be better.
Salts should be unique per user. This is so that when two users have the same password, the hashes will be different. This prevents a variety of attacks, like rainbow tables or looking at the frequency of hashes. Having a constant value appended to the password doesn't make these attacks any more difficult.