Add signed invites, hang links, multi-share, and EXTENSIONS.md

- Signed invites: waste: URI gains inviter+sig fields (Ed25519); hello
  carries the invite so receiving peers can verify against known keys
- RequireInvite per-network flag: rejects peers without valid signed invite
- Hash-based hang links: #waste:base64 fragment pre-fills join form without
  server-side leakage of network name
- Multi-share: shares.json (daemon) + waste_shares localStorage (browser);
  IPC add_share/remove_share/list_shares commands
- EXTENSIONS.md: addendum documenting all waste-go protocol deviations from
  YAW/2; all extensions are additive and backward compatible

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/tui/main.go

@@ -283,7 +283,7 @@ func (m model) applyEvent(evt proto.IpcMessage) model {
 		}
 
 	case proto.EvtInviteGenerated:
-		m.invitePopup = evt.InviteString
+		m.invitePopup = evt.InviteGenerated
 
 	case proto.EvtMessageReceived:
 		if evt.Message != nil {