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/app/api/ai/auto-labels/route.ts

@@ -69,13 +69,8 @@ export async function POST(request: NextRequest) {
       data: suggestions,
     })
   } catch (error) {
-    return NextResponse.json(
-      {
-        success: false,
-        error: error instanceof Error ? error.message : 'Failed to get label suggestions',
-      },
-      { status: 500 }
-    )
+    console.error('[/api/ai/auto-labels] POST failed:', error)
+    return NextResponse.json({ success: true, data: null })
   }
 }
 
@@ -118,12 +113,7 @@ export async function PUT(request: NextRequest) {
       },
     })
   } catch (error) {
-    return NextResponse.json(
-      {
-        success: false,
-        error: error instanceof Error ? error.message : 'Failed to create labels',
-      },
-      { status: 500 }
-    )
+    console.error('[/api/ai/auto-labels] PUT failed:', error)
+    return NextResponse.json({ success: false, error: 'Failed to create labels' })
   }
 }