Activation errors now show the failing node name

When a workflow fails to activate because of a node-level error, users can now immediately identify which node caused the problem. The system displays the node name in the error toast and includes the technical error description in a collapsible "Show details" section — the same pattern used for workflow execution errors. This makes debugging faster and more consistent across the platform. The error information flows from the backend through the push event system to the frontend, with the node ID resolved to a human-readable name using a new composable. A minor refactor updates the toast notification API to accept an options object instead of positional arguments, improving clarity for the 15+ call sites that display error messages.