Added SkillTreeType enum and implemented dropdown selection in SkillTreeView

- Introduced a new `SkillTreeType` enum to represent different skill tree categories: General, Coding, Data, and Scrape/Synthesize.
- Extended the `SkillTreeType` enum to provide associated string values and JSON file names for each category.
- Refactored the `SkillTreeViewModel` to reload the skill tree data based on the selected category.
- Enhanced `SkillTreeView` by adding a positioned dropdown in the top-left corner to allow users to select and load different skill tree categories dynamically.

frontend/lib/models/skill_tree/skill_tree_category.dart

@@ -0,0 +1,38 @@
+enum SkillTreeCategory {
+  general,
+  coding,
+  data,
+  scrapeSynthesize,
+}
+
+extension SkillTreeTypeExtension on SkillTreeCategory {
+  String get stringValue {
+    switch (this) {
+      case SkillTreeCategory.general:
+        return 'General';
+      case SkillTreeCategory.coding:
+        return 'Coding';
+      case SkillTreeCategory.data:
+        return 'Data';
+      case SkillTreeCategory.scrapeSynthesize:
+        return 'Scrape/Synthesize';
+      default:
+        return '';
+    }
+  }
+
+  String get jsonFileName {
+    switch (this) {
+      case SkillTreeCategory.general:
+        return 'general_tree_structure.json';
+      case SkillTreeCategory.coding:
+        return 'coding_tree_structure.json';
+      case SkillTreeCategory.data:
+        return 'data_tree_structure.json';
+      case SkillTreeCategory.scrapeSynthesize:
+        return 'scrape_synthesize_tree_structure.json';
+      default:
+        return '';
+    }
+  }
+}