[ HiveTalk/swarm ] Fix npub conversion by sending hex from frontend

- Frontend now sends converted hex pubkey instead of raw input

- Prevents server-side bech32 decoding issues

- Maintains fallback npub conversion on server for robustness

- Fixes 'invalid padding' error in bech32 implementation

The frontend successfully converts npub to hex using nostr-tools,

so we send that validated hex value to the server.

https://github.com/HiveTalk/swarm/commit/aaba6f67bbf5ed2d48b6a76b78a813aacfea1fcd