MVP: Wire canvas persistence to Node API

## Goal
Connect frontend canvas state to backend node persistence.

## Scope
- Initial load: fetch nodes and render from API
- Persist create/move/update operations
- Handle API failures gracefully in UI
- Avoid data loss on refresh

## Acceptance Criteria
- Canvas reload restores persisted nodes
- Node changes are reflected in backend data
- Errors are visible and actionable for users
