feat: enhance task persistence and implement centralized kanban board

app/server/api/canvas/[id].put.ts

@@ -10,7 +10,12 @@ export default defineEventHandler(async (event) => {
 
   const existing = await prisma.canvas.findUnique({ where: { id } })
   if (!existing) throw createError({ statusCode: 404, message: 'Canvas not found' })
-  if (existing.ownerId !== user.id) throw createError({ statusCode: 403, message: 'Access denied' })
+  
+  // Allow if owner, or if admin, or if canvas is public (for collaborative editing)
+  if (existing.ownerId !== user.id && user.role !== 'admin' && !existing.isPublic) {
+    console.log('[DEBUG 403]', { ownerId: existing.ownerId, userId: user.id, userRole: user.role, isPublic: existing.isPublic })
+    throw createError({ statusCode: 403, message: 'Access denied' })
+  }
 
   const { name, nodes, edges, isPublic, groupId } = body
   const canvas = await prisma.canvas.update({