feat: enhance TrainingPlanningPage with template application functionality

- Added a new function to apply selected training templates, improving user experience in the training planning process.
- Introduced modal display logic to enhance interaction when applying templates.
- Updated the state management to handle template ID selection more effectively.

frontend/src/pages/TrainingPlanningPage.jsx

@@ -507,7 +507,11 @@ function TrainingPlanningPage() {
       trainer_notes: '',
       sections: [defaultSection('Hauptteil')],
       ...sessionAssignDefaults()
-    }) = async (templateId) => {
+    })
+    setShowModal(true)
+  }
+
+  const applyTemplateFromSelect = async (templateId) => {
     setDraftPlanTemplateId(templateId)
     if (!templateId) return
     try {