refactor: extract BLE protocol stack into meshcore_client package

Move BLE communication layer (command queue, frame parser/builder,
protocol constants, data models) into a standalone reusable Dart package
at ../meshcore_client. App model files become thin re-export wrappers,
keeping all existing consumers working without import changes.

pubspec.yaml

@@ -40,6 +40,10 @@ dependencies:
   # Use with the CupertinoIcons class for iOS style icons.
   cupertino_icons: ^1.0.8
 
+  # MeshCore BLE protocol client
+  meshcore_client:
+    path: ../meshcore_client
+
   # BLE connectivity
   flutter_blue_plus: ^2.0.0