feat(display): Sprint 7 — USB serial link to AR-Concentrador via NMEA $PARP

- Add flutter_libserialport dependency (pubspec.yaml)
- New ParpCodec: XOR-checksum NMEA parser + command builders for all PARP sentences
- New ConcentradorService: manages two independent COM ports (RX-OUT broadcast,
  TX-IN commands) at 115200/8N1; auto-fires onConnectionChanged on link drop
- AutopilotState: dual-mode operation (demo timer OR live serial); connectToSerial /
  disconnectSerial; command methods (engage/disengage/adjustSetpoint) forward to
  ConcentradorService when connected; falls back to demo on disconnect
- New PortSettingsScreen (/settings/ports): RX+TX dropdowns populated from
  SerialPort.availablePorts, persisted in SharedPreferences; Connect/Disconnect
  buttons with error display and snackbar feedback
- main.dart: auto-connect to saved ports on startup (silent fail → demo mode);
  registers /settings/ports route
- CockpitScreen: gear icon replaced with PopupMenuButton (Puertos COM / Apariencia)

AR_electronics — AR-Autopilot Project

display/lib/screens/cockpit/cockpit_screen.dart

@@ -31,6 +31,7 @@ import '../../widgets/themed/mode_selector.dart';
 import '../../widgets/themed/rudder_indicator.dart';
 import '../../widgets/themed/status_chip.dart';
 import '../settings/appearance_settings.dart';
+import '../settings/port_settings_screen.dart';
 
 class CockpitScreen extends StatelessWidget {
   const CockpitScreen({super.key});
@@ -138,14 +139,47 @@ class _TopBar extends StatelessWidget {
             status: ap.isConnected ? StatusLevel.ok : StatusLevel.warn,
           ),
           const SizedBox(width: 16),
-          GestureDetector(
-            onTap: () => Navigator.pushNamed(
-                context, AppearanceSettingsScreen.routeName),
-            child: Icon(
+          PopupMenuButton<String>(
+            icon: Icon(
               Icons.settings_outlined,
               color: theme.textMuted,
               size: 22,
             ),
+            color: theme.backgroundMid,
+            shape: RoundedRectangleBorder(
+              borderRadius: BorderRadius.circular(8),
+              side: BorderSide(color: theme.panelBorder),
+            ),
+            onSelected: (route) => Navigator.pushNamed(context, route),
+            itemBuilder: (_) => [
+              PopupMenuItem(
+                value: PortSettingsScreen.routeName,
+                child: Row(
+                  children: [
+                    Icon(Icons.usb, color: theme.accentMid, size: 18),
+                    const SizedBox(width: 10),
+                    Text(
+                      'Puertos COM',
+                      style: TextStyle(color: theme.textMain, fontSize: 13),
+                    ),
+                  ],
+                ),
+              ),
+              PopupMenuItem(
+                value: AppearanceSettingsScreen.routeName,
+                child: Row(
+                  children: [
+                    Icon(Icons.palette_outlined,
+                        color: theme.accentMid, size: 18),
+                    const SizedBox(width: 10),
+                    Text(
+                      'Apariencia',
+                      style: TextStyle(color: theme.textMain, fontSize: 13),
+                    ),
+                  ],
+                ),
+              ),
+            ],
           ),
         ],
       ),