major bug fixes

lib/remote/host_server.dart

@@ -116,6 +116,11 @@ class RemoteHost {
   // ---- Handshake + per-client loop --------------------------------------
 
   void _handleSocket(WebSocket socket) {
+    // Keepalive: ping idle clients so a controller that vanished without a
+    // close frame (crashed, walked out of Wi-Fi range) is detected and pruned
+    // instead of lingering as a zombie in `_clients`.
+    socket.pingInterval = const Duration(seconds: 15);
+
     final nonce = RemoteAuth.newNonce();
     var authed = false;