feat: Show selected node details in the side panel

[lornakelly]

Dependency on sidepanel implementation: #144

Description

Surface the selected node's information in the side panel. When a node is selected in the diagram, the side panel should open (if not already visible) and display the details of that node from the underlying workflow model. Changing the selection should update the panel contents accordingly, and clearing the selection should reset the panel to its default/empty state and close

Motivation

Selection highlighting tells the user which node is active, but not what it is. Wiring the side panel to the current selection gives users immediate visibility into a node without needing to open the YAML/JSON, making the editor easier to navigate and inspect, especially for larger workflows.

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.