feat: Phase B - Frontend API für M:N Zielgruppen

Frontend API Layer:
- 5 neue api.js Funktionen für M:N Management
- listTrainingStyleTargetGroups(filters) - Liste mit Filtern
- createTrainingStyleTargetGroup(data) - Neue Zuordnung
- updateTrainingStyleTargetGroup(id, data) - is_primary ändern
- deleteTrainingStyleTargetGroup(id) - Zuordnung löschen
- getTrainingStylesHierarchy() - Hierarchische Struktur für Tree-View

Pattern: Konsistent mit bestehenden Catalog-Funktionen
Export: Alle Funktionen zum api-Objekt hinzugefügt

Version: 0.3.4 (Phase B)

frontend/src/utils/api.js

@@ -370,6 +370,35 @@ export async function deleteTargetGroup(id) {
   return request(`/api/target-groups/${id}`, { method: 'DELETE' })
 }
 
+// Training Style → Target Groups (M:N Assignments)
+export async function listTrainingStyleTargetGroups(filters = {}) {
+  const query = new URLSearchParams(filters).toString()
+  return request(`/api/training-style-target-groups${query ? '?' + query : ''}`)
+}
+
+export async function createTrainingStyleTargetGroup(data) {
+  return request('/api/training-style-target-groups', {
+    method: 'POST',
+    body: JSON.stringify(data)
+  })
+}
+
+export async function updateTrainingStyleTargetGroup(id, data) {
+  return request(`/api/training-style-target-groups/${id}`, {
+    method: 'PUT',
+    body: JSON.stringify(data)
+  })
+}
+
+export async function deleteTrainingStyleTargetGroup(id) {
+  return request(`/api/training-style-target-groups/${id}`, { method: 'DELETE' })
+}
+
+export async function getTrainingStylesHierarchy(filters = {}) {
+  const query = new URLSearchParams(filters).toString()
+  return request(`/api/training-styles/hierarchy${query ? '?' + query : ''}`)
+}
+
 // ============================================================================
 // Training Planning
 // ============================================================================
@@ -494,6 +523,11 @@ export const api = {
   createTargetGroup,
   updateTargetGroup,
   deleteTargetGroup,
+  listTrainingStyleTargetGroups,
+  createTrainingStyleTargetGroup,
+  updateTrainingStyleTargetGroup,
+  deleteTrainingStyleTargetGroup,
+  getTrainingStylesHierarchy,
 
   // System
   getVersion,