Add SkillTreeEdge data model for skill tree

The SkillTreeEdge model represents the relationship between different skill nodes.
It includes:
- Edge ID
- Source node ID
- Destination node ID
- Arrows property to indicate directionality

frontend/lib/models/skill_tree/skill_tree_edge.dart

@@ -0,0 +1,23 @@
+class SkillTreeEdge {
+  final String id;
+  final String from;
+  final String to;
+  final String arrows;
+
+  SkillTreeEdge({
+    required this.id,
+    required this.from,
+    required this.to,
+    required this.arrows,
+  });
+
+  // Optionally, add a factory constructor to initialize from JSON
+  factory SkillTreeEdge.fromJson(Map<String, dynamic> json) {
+    return SkillTreeEdge(
+      id: json['id'],
+      from: json['from'],
+      to: json['to'],
+      arrows: json['arrows'],
+    );
+  }
+}