Refactor terminal and remove duplicate code.

lib/terminal/models/body_chunk.dart

@@ -0,0 +1,7 @@
+class BodyChunk {
+  BodyChunk({required this.ts, required this.text, required this.sizeBytes});
+
+  final DateTime ts;
+  final String text; // preview text (could be partial)
+  final int sizeBytes;
+}