fix: nip-44 incompatibility with other clients

[Punk7149]

Summary

Previous implementation doesn't work correctly. Tested e.g. with Amethyst and nak. The kind 4 messages encrypted with nip44 could not be decrypted.

What changed?

Replaced the current implementation with partially ported / refactored reference kotlin implementation.

BREAKING

Messages with previous implementation cannot be decrypted.

Review focus

Checklist

• [x] Scope ≤ 300 lines (or split/stack)
• [x] Title is verb + object (e.g., “Refactor auth middleware to async”)
• [x] Description links the issue and answers “why now?”
• [x] BREAKING flagged if needed
• [x] Tests/docs updated (if relevant)