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/main.dart

@@ -1,25 +1,48 @@
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 
 import 'data/autopilot_state.dart';
 import 'theme/theme_provider.dart';
 import 'screens/cockpit/cockpit_screen.dart';
 import 'screens/settings/appearance_settings.dart';
+import 'screens/settings/port_settings_screen.dart';
+
+// SharedPreferences keys — must match port_settings_screen.dart
+const _kRxKey = 'port.rx';
+const _kTxKey = 'port.tx';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
+
+  // Load persisted theme before first frame.
   final themeProvider = await AutopilotThemeProvider.load();
+
+  // Create state object early so we can attempt auto-connect.
+  final autopilotState = AutopilotState();
+
+  // Attempt to reconnect to the last-used COM ports silently.
+  // If the ports are not available (hardware unplugged, different PC, etc.)
+  // the exception is swallowed and the UI stays in demo mode.
+  try {
+    final prefs = await SharedPreferences.getInstance();
+    final rxPort = prefs.getString(_kRxKey);
+    final txPort = prefs.getString(_kTxKey);
+    if (rxPort != null && txPort != null) {
+      await autopilotState.connectToSerial(rxPort: rxPort, txPort: txPort);
+    }
+  } catch (_) {
+    // Hardware not available — stay in demo mode.
+  }
+
   runApp(
     MultiProvider(
       providers: [
         ChangeNotifierProvider<AutopilotThemeProvider>.value(
           value: themeProvider,
         ),
-        // AutopilotState drives the cockpit UI.
-        // Sprint 4: internal demo timer (vessel simulation).
-        // Sprint 7: replace with AutopilotStateModbus or wire Modbus RTU here.
-        ChangeNotifierProvider<AutopilotState>(
-          create: (_) => AutopilotState(),
+        ChangeNotifierProvider<AutopilotState>.value(
+          value: autopilotState,
         ),
       ],
       child: const ArAutopilotApp(),
@@ -38,9 +61,9 @@ class ArAutopilotApp extends StatelessWidget {
       theme: ThemeData(useMaterial3: true),
       initialRoute: CockpitScreen.routeName,
       routes: {
-        CockpitScreen.routeName: (_) => const CockpitScreen(),
-        AppearanceSettingsScreen.routeName: (_) =>
-            const AppearanceSettingsScreen(),
+        CockpitScreen.routeName:         (_) => const CockpitScreen(),
+        AppearanceSettingsScreen.routeName: (_) => const AppearanceSettingsScreen(),
+        PortSettingsScreen.routeName:    (_) => const PortSettingsScreen(),
       },
     );
   }