fix(test): remove unnecessary flutter/material.dart import (#120)

Pre-existing analyzer hint in graph_painter_test.dart — flutter_test
already re-exports material.dart. Fixes CI failure.

Co-authored-by: Claude <noreply@anthropic.com>

Author: nickmeinhold

test/ui/graph/graph_painter_test.dart

@@ -4,7 +4,6 @@ import 'package:engram/src/models/relationship.dart';
 import 'package:engram/src/ui/graph/graph_edge.dart';
 import 'package:engram/src/ui/graph/graph_node.dart';
 import 'package:engram/src/ui/graph/graph_painter.dart';
-import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 GraphNode _node(String id, {Offset position = Offset.zero}) {