refactor(node): QUIC does not relay chat it cannot check

The QUIC handler stored `payload` as it arrived and broadcast it: no envelope,
no signature check, no `device_hello` to check one against. A message reaching
a group's archive that way is a plaintext row in an encrypted history, and it
would be indistinguishable from one somebody actually wrote.

Removed rather than gated. The transport implements neither the per-device
sealing nor the device identification the WebRTC path requires, so refusing
here would mean maintaining a second, weaker set of rules for a transport with
no client; an unimplemented type is logged and dropped, like every other message
this transport does not have.

The comment on the peer registry loses its chat fan-out aside for the same
reason.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

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