Add ability to send images over lora

test/services/image_codec_png_test.dart

@@ -0,0 +1,47 @@
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_open/services/image_codec_service.dart';
+
+/// Covers the one piece of the decode path that can be executed without a model
+/// file, a device or ONNX Runtime: turning the backend's packed RGB output into
+/// PNG bytes a widget can render.
+///
+/// It needs the engine's image codecs, hence the binding.
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  group('ImageCodecService.rgbToPng', () {
+    test('encodes a PNG with the right magic bytes and dimensions', () async {
+      const side = 8;
+      final rgb = Uint8List(side * side * 3);
+      for (var i = 0; i < rgb.length; i++) {
+        rgb[i] = i & 0xFF;
+      }
+
+      final png = await ImageCodecService.rgbToPng(rgb, side);
+
+      expect(png.sublist(0, 8), [
+        0x89,
+        0x50,
+        0x4E,
+        0x47,
+        0x0D,
+        0x0A,
+        0x1A,
+        0x0A,
+      ]);
+      // IHDR width/height, big-endian at offsets 16 and 20.
+      final header = ByteData.sublistView(png);
+      expect(header.getUint32(16), side);
+      expect(header.getUint32(20), side);
+    });
+
+    test('rejects a buffer that is not RGB at the stated size', () async {
+      await expectLater(
+        ImageCodecService.rgbToPng(Uint8List(10), 8),
+        throwsA(isA<ArgumentError>()),
+      );
+    });
+  });
+}