Introduce `NotFoundPage`, update routing, and streamline workflow handling:

- Add a user-friendly 404 page with navigation options.
- Update `App` routing to include a fallback route for undefined paths.
- Overhaul WorkflowsPage by removing snapshot handling and redundant methods.
- Simplify DiagramEditor and CanvasDiagramEditor by dropping snapshot-related state.
- Enhance `WorkflowDetailsPage` with better structure and clean up unnecessary logic.
- Refactor step-related services and types for better consistency and readability.