fix(relay-members): route NIP-43 join/leave through launchSigner

The join/leave-request buttons launched a raw scope.launch(Dispatchers.IO)
and called account.signer.sign() directly. When the signer threw —
TimedOutException from a Samsung-A53 / Android-16 Amber prompt the user
didn't respond to in 30s, or any other SignerException — the failure
escaped the composable's scope and was reported as an uncaught crash
instead of being toasted/logged like every other signing operation.

Route both buttons through accountViewModel.launchSigner so the same
SignerExceptions handling that every other signing entry point uses
applies here too.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/relays/nip43/RelayMembersScreen.kt

@@ -154,13 +154,13 @@ fun RelayMembersScreen(
                 joinRequestSent = joinRequestSent,
                 leaveRequestSent = leaveRequestSent,
                 onJoinRequest = {
-                    scope.launch(Dispatchers.IO) {
+                    accountViewModel.launchSigner {
                         sendJoinRequest(normalizedRelayUrl, accountViewModel)
                         joinRequestSent = true
                     }
                 },
                 onLeaveRequest = {
-                    scope.launch(Dispatchers.IO) {
+                    accountViewModel.launchSigner {
                         sendLeaveRequest(normalizedRelayUrl, accountViewModel)
                         leaveRequestSent = true
                     }