feat: Implement single node selection and highlight

[lornakelly]

Description

Implement single node selection within the diagram. Clicking a node should mark it as selected and apply a visual highlight to indicate its state. Only one node can be selected at a time, with any previous selection cleared on new selection.

Motivation

Improve usability and interaction by providing clear visual feedback for the currently selected node.

Proposed Implementation

No response

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.