feat: Integrate ELKjs for automatic diagram layout

[lornakelly]

Description

Integrate ELKjs as the auto-layout engine for the diagram.

Motivation

ELKjs will calculate meaningful positions for nodes and edges automatically, making the diagram readable for any workflow.

Proposed Implementation

• The implementation should be framework-agnostic and sit outside the React Flow layer.
• It needs nodes, edges, node dimensions, and parent/child relationships as input
• For this first iteration, out of the box ELK behavior is sufficient - no custom layout configuration is required.
• Replace previous hardcoded positions in array with ELK calculated positions

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.