fix: label management - transaction safety, deletion sync, error handling

- Use prisma.$transaction in auto-label-creation.service with tx client
- Fix DELETE /api/labels to properly JSON.parse + disconnect labelRelations
- Fix PUT /api/labels rename to handle JSON labels
- Graceful error handling in /api/ai/tags and /api/ai/auto-labels
- Client-side label-deleted event in home-client, notes-tabs-view, label-management-dialog

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/components/label-management-dialog.tsx

@@ -50,8 +50,12 @@ export function LabelManagementDialog(props: LabelManagementDialogProps = {}) {
 
   const handleDeleteLabel = async (id: string) => {
     try {
+      const labelToDelete = labels.find(l => l.id === id)
       await deleteLabel(id)
       triggerRefresh()
+      if (labelToDelete) {
+        window.dispatchEvent(new CustomEvent('label-deleted', { detail: { name: labelToDelete.name } }))
+      }
       setConfirmDeleteId(null)
     } catch (error) {
       console.error('Failed to delete label:', error)