Update Skill Tree Picker and Catalog Functions for Clarity

- Modified comments in `SkillTreePickerPanel` and `skillCatalogTree.js` to accurately reflect the default expansion behavior, specifying that only main groups are open by default.
- Refactored `defaultExpandedKeysForSkillTree` to simplify the logic, ensuring only main groups are returned as expanded.
- Adjusted tests in `skillCatalogTree.test.js` to validate the updated behavior, confirming that only main groups are opened in the skill tree.

frontend/src/components/SkillTreePickerPanel.jsx

@@ -78,7 +78,7 @@ function SkillTreeNodes({ nodes, depth, expanded, exclude, onToggle, onPickSkill
 }
 
 /**
- * Ausklappbare Baumliste: Hauptgruppe → Kategorie → Fähigkeit (Hauptgruppe und Kategorie standard offen).
+ * Ausklappbare Baumliste: Hauptgruppe → Kategorie → Fähigkeit (nur Hauptgruppe standard offen).
  */
 export default function SkillTreePickerPanel({
   skills = [],

frontend/src/utils/skillCatalogTree.js

@@ -162,19 +162,10 @@ export function allExpandableKeys(tree) {
   return keys
 }
 
-/** Standard: Hauptgruppe und Kategorie aufgeklappt (Fähigkeiten direkt darunter sichtbar). */
+/** Standard: nur Hauptgruppe aufgeklappt; Kategorien und Fähigkeiten zugeklappt. */
 export function defaultExpandedKeysForSkillTree(tree) {
-  const keys = []
-  const walk = (nodes) => {
-    for (const n of nodes) {
-      if (n.type === 'main' || n.type === 'category') {
-        keys.push(n.key)
-        walk(n.children || [])
-      }
-    }
-  }
-  walk(tree)
-  return keys
+  if (!Array.isArray(tree)) return []
+  return tree.filter((n) => n.type === 'main').map((n) => n.key)
 }
 
 /** Sentinel für Katalog-Admin: keine Hauptgruppe gewählt. */

frontend/src/utils/skillCatalogTree.test.js

@@ -49,11 +49,11 @@ describe('skillCatalogTree', () => {
     expect(kataSkill.skillId).toBe(2)
   })
 
-  it('defaultExpandedKeysForSkillTree opens main and category only', () => {
+  it('defaultExpandedKeysForSkillTree opens main groups only', () => {
     const tree = buildSkillCatalogTree(sample)
     const keys = defaultExpandedKeysForSkillTree(tree)
-    expect(keys).toContain('m-10')
-    expect(keys.some((k) => k.includes('c-21'))).toBe(true)
+    expect(keys).toEqual(['m-10'])
+    expect(keys.some((k) => k.includes('c-'))).toBe(false)
     expect(keys.some((k) => k.startsWith('s-'))).toBe(false)
   })