feat: USB serial companion support (Android)

- Add MeshCoreSerialService to meshcore_client (0x3C/0x3E framing)
- Add usb_serial package for Android USB OTG
- Add UsbSerialTransport wrapper for Android
- Add USB tab to connection dialog (scan, select, connect)
- Add ConnectionMode.usb + connectSerial/disconnectSerial
- Web Serial stub (not yet implemented)
- Fix message limits: DM 156→150, Channel 127→160 (firmware MAX_TEXT_LEN=160)

lib/services/serial/usb_serial_transport.dart

@@ -0,0 +1,101 @@
+import 'dart:async';
+import 'dart:typed_data';
+import 'package:flutter/foundation.dart';
+import 'package:usb_serial/usb_serial.dart';
+import 'package:meshcore_client/meshcore_client.dart';
+
+/// Android USB OTG transport for MeshCore serial connection.
+///
+/// Wraps the `usb_serial` package to provide raw byte I/O to
+/// [MeshCoreSerialService].
+class UsbSerialTransport {
+  static const int _baudRate = 115200;
+
+  UsbPort? _port;
+  StreamSubscription? _inputSubscription;
+  final MeshCoreSerialService _service;
+
+  UsbSerialTransport(this._service);
+
+  MeshCoreSerialService get service => _service;
+
+  bool get isConnected => _port != null;
+
+  /// List available USB serial devices.
+  static Future<List<UsbDevice>> listDevices() async {
+    return UsbSerial.listDevices();
+  }
+
+  /// Connect to a USB serial device.
+  Future<bool> connect(UsbDevice device) async {
+    try {
+      _port = await device.create();
+      if (_port == null) {
+        debugPrint('❌ [USB] Failed to create port for ${device.productName}');
+        return false;
+      }
+
+      final opened = await _port!.open();
+      if (!opened) {
+        debugPrint('❌ [USB] Failed to open port');
+        _port = null;
+        return false;
+      }
+
+      await _port!.setDTR(true);
+      await _port!.setRTS(true);
+      await _port!.setPortParameters(
+        _baudRate,
+        UsbPort.DATABITS_8,
+        UsbPort.STOPBITS_1,
+        UsbPort.PARITY_NONE,
+      );
+
+      // Wire write callback
+      _service.writeRaw = _write;
+
+      // Listen for incoming data
+      _inputSubscription = _port!.inputStream?.listen(
+        (data) => _service.feedRawBytes(data),
+        onError: (error) {
+          debugPrint('❌ [USB] Read error: $error');
+          disconnect();
+        },
+        onDone: () {
+          debugPrint('⚠️ [USB] Port closed');
+          disconnect();
+        },
+      );
+
+      debugPrint(
+        '✅ [USB] Connected to ${device.productName} at $_baudRate baud',
+      );
+
+      // Initialize MeshCore session
+      return _service.markConnected();
+    } catch (e) {
+      debugPrint('❌ [USB] Connection failed: $e');
+      await disconnect();
+      return false;
+    }
+  }
+
+  /// Disconnect from the USB device.
+  Future<void> disconnect() async {
+    _service.markDisconnected();
+    _service.writeRaw = null;
+    await _inputSubscription?.cancel();
+    _inputSubscription = null;
+    await _port?.close();
+    _port = null;
+  }
+
+  Future<void> _write(Uint8List data) async {
+    if (_port == null) throw Exception('USB not connected');
+    await _port!.write(data);
+  }
+
+  void dispose() {
+    disconnect();
+  }
+}

lib/services/serial/web_serial_transport.dart

@@ -0,0 +1,22 @@
+import 'dart:async';
+import 'dart:typed_data';
+import 'package:flutter/foundation.dart';
+import 'package:meshcore_client/meshcore_client.dart';
+
+// Web Serial API is only available on web platform.
+// This file provides the interface; the actual JS interop is conditional.
+
+/// Web Serial transport stub for non-web platforms.
+/// On web, use [WebSerialTransportImpl] from web_serial_transport_web.dart.
+class WebSerialTransport {
+  final MeshCoreSerialService service;
+
+  WebSerialTransport(this.service);
+
+  bool get isConnected => false;
+  bool get isSupported => false;
+
+  Future<bool> requestAndConnect() async => false;
+  Future<void> disconnect() async {}
+  void dispose() {}
+}