Uses the existing gift-wrap primitive (kind 14 rumor, one p-tag per recipient):
- useGroupConversations: lists multi-recipient threads, bucketed by the sorted
participant key; the inbox shows a Groups section
- GroupConversationPage (/messages/group/:participants): encrypted group thread
with sender names, send + error feedback
- smoke test now boots the new route
Group creation UI (multi-select in NewMessageDialog) is the next step. For
stronger guarantees (forward secrecy, rosters) Marmot/MLS or Cordn would be the
protocol to adopt later; noted in the hook.
