feat: Configure default public channel with well-known secret and enhance channel message handling

lib/services/meshcore_ble_service.dart

@@ -315,6 +315,13 @@ class MeshCoreBleService {
     _responseHandler.trackSentMessage(messageId, null);
   }
 
+  /// Send a text message to a channel (flood-mode broadcast)
+  ///
+  /// Channel messages are ephemeral and use flood routing (no ACKs).
+  /// Use channel 0 for the default public channel.
+  ///
+  /// Note: Uses fire-and-forget mode since channel messages don't return
+  /// delivery confirmation (they're broadcast to all nodes).
   Future<void> sendChannelMessage({
     required int channelIdx,
     required String text,
@@ -324,6 +331,8 @@ class MeshCoreBleService {
       throw ArgumentError('Channel message too long (max ~160 characters)');
     }
 
+    // Channel messages use fire-and-forget (no ACK expected)
+    // The firmware responds with RESP_CODE_OK but we don't wait for it
     await _commandSender.writeData(FrameBuilder.buildSendChannelTxtMsg(
       channelIdx: channelIdx,
       text: text,
@@ -483,20 +492,26 @@ class MeshCoreBleService {
     await _commandSender.writeData(FrameBuilder.buildGetChannel(channelIdx));
   }
 
-  /// Set the name for a specific channel
+  /// Set the name and secret for a specific channel
+  ///
+  /// The secret must be exactly 16 bytes (128-bit encryption key).
+  /// For the default public channel (channel 0), use [MeshCoreConstants.defaultPublicChannelSecret].
   Future<void> setChannel({
     required int channelIdx,
     required String channelName,
+    required List<int> secret,
   }) async {
-    debugPrint('📻 [BLE] Setting channel name:');
+    debugPrint('📻 [BLE] Setting channel:');
     debugPrint('    Channel index: $channelIdx');
     debugPrint('    Channel name: $channelName');
+    debugPrint('    Secret length: ${secret.length} bytes');
 
     await _commandSender.writeDataAndWaitForAck(FrameBuilder.buildSetChannel(
       channelIdx: channelIdx,
       channelName: channelName,
+      secret: secret,
     ));
-    debugPrint('✅ [BLE] CMD_SET_CHANNEL sent');
+    debugPrint('✅ [BLE] CMD_SET_CHANNEL sent successfully');
   }
 
   /// Sync all channels from the device (typically 0-39)

lib/services/meshcore_constants.dart

@@ -117,6 +117,16 @@ class MeshCoreConstants {
   static const int binaryReqGetAccessList = 0x05;
   static const int binaryReqGetNeighbours = 0x06;
 
+  // Default Public Channel Secret (128-bit)
+  // This is the well-known pre-shared key for the public channel (channel 0)
+  // Hex: 8b3387e9c5cdea6ac9e5edbaa115cd72
+  // Base64: izOH6cXN6mrJ5e26oRXNcg==
+  // Source: https://github.com/meshcore-dev/MeshCore/blob/main/docs/faq.md
+  static const List<int> defaultPublicChannelSecret = [
+    0x8b, 0x33, 0x87, 0xe9, 0xc5, 0xcd, 0xea, 0x6a,
+    0xc9, 0xe5, 0xed, 0xba, 0xa1, 0x15, 0xcd, 0x72,
+  ];
+
   // Cayenne LPP Data Types
   static const int lppDigitalInput = 0;
   static const int lppDigitalOutput = 1;

lib/services/protocol/frame_builder.dart

@@ -252,11 +252,19 @@ class FrameBuilder {
     return writer.toBytes();
   }
 
-  /// Build SetChannel command - sets the name for a specific channel
+  /// Build SetChannel command - sets the name and secret for a specific channel
+  ///
+  /// Format: [cmd(1)][channel_idx(1)][name(32)][secret(16)]
+  /// Secret must be exactly 16 bytes (128-bit key)
   static Uint8List buildSetChannel({
     required int channelIdx,
     required String channelName,
+    required List<int> secret,
   }) {
+    if (secret.length != 16) {
+      throw ArgumentError('Channel secret must be exactly 16 bytes (got ${secret.length})');
+    }
+
     final writer = BufferWriter();
     writer.writeByte(MeshCoreConstants.cmdSetChannel); // 0x20 (32)
     writer.writeByte(channelIdx); // 0-39 typically
@@ -268,6 +276,9 @@ class FrameBuilder {
     nameBytes.setRange(0, copyLen, encoded);
     writer.writeBytes(nameBytes);
 
+    // Write 16-byte secret
+    writer.writeBytes(Uint8List.fromList(secret));
+
     return writer.toBytes();
   }
 }