Add Info data model for skill tree

The Info data model holds metadata about a skill node.
It includes:
- The difficulty level of the skill node
- A description of the skill node
- A list of potential side effects related to the skill node

frontend/lib/models/skill_tree/info.dart

@@ -0,0 +1,19 @@
+class Info {
+  final String difficulty;
+  final String description;
+  final List<String> sideEffects;
+
+  Info({
+    required this.difficulty,
+    required this.description,
+    required this.sideEffects,
+  });
+
+  factory Info.fromJson(Map<String, dynamic> json) {
+    return Info(
+      difficulty: json['difficulty'],
+      description: json['description'],
+      sideEffects: List<String>.from(json['side_effects']),
+    );
+  }
+}