Even with a strong password – my laptop today can do 1100 MH/s which makes it quite feasible. One of the potential attack vectors can look like this: a malicious actor takes this and reskins the UI, then hosts and does the marketing, his server stores the NGINX logs with /login requests for some time, preserving the password_hash and username params silently. Then after a while he spends few weeks cracking the passwords for every account and syncs their notes, then parsing them for passwords/sensitive info.

As for suggestions, this could be enforced by making use of a system-level keychain, but I don't know if it is possible within a sandboxed environment like PWA. Some stronger derivation method would help too, like the industry standard is pbkdf2 with at least 600000 iterations for SHA256. Lastly, end-to-end encryption usually implies some kind of protection against man-in-the-middle attack, like a certificate pinning mechanism