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/lib/ai/services/auto-label-creation.service.ts

@@ -431,14 +431,16 @@ Deine Antwort (nur JSON):
   ): Promise<number> {
     let createdCount = 0
 
+    await prisma.$transaction(async (tx) => {
     for (const suggestedLabel of suggestions.suggestedLabels) {
       if (!selectedLabels.includes(suggestedLabel.name)) continue
 
-      // Create the label
-      const label = await prisma.label.create({
-        data: {
+      const label = await tx.label.upsert({
+        where: { notebookId_name: { notebookId, name: suggestedLabel.name } as any },
+        update: {},
+        create: {
           name: suggestedLabel.name,
-          color: 'gray', // Default color, user can change later
+          color: 'gray',
           notebookId,
           userId,
         },
@@ -446,7 +448,7 @@ Deine Antwort (nur JSON):
 
       // Assign to notes: UI reads `Note.labels` (JSON string[]); relations must stay in sync
       for (const noteId of suggestedLabel.noteIds) {
-        const note = await prisma.note.findFirst({
+        const note = await tx.note.findFirst({
           where: { id: noteId, userId, notebookId },
           select: { labels: true },
         })
@@ -469,7 +471,7 @@ Deine Antwort (nur JSON):
           names = [...names, suggestedLabel.name]
         }
 
-        await prisma.note.update({
+        await tx.note.update({
           where: { id: noteId },
           data: {
             labels: JSON.stringify(names),
@@ -482,6 +484,7 @@ Deine Antwort (nur JSON):
 
       createdCount++
     }
+    })
 
     return createdCount
   }