feat: Style start/end node components #79

Description

The core implementation of Start and End node components in the editor are complete. This ticket is to apply the agreed styling and test to make sure edges are rendering correctly and connecting these custom nodes with standard nodes. Verify by testing multiple workflows and confirm these nodes are connected to the correct start and end task nodes.

Motivation

Improve diagram readability and user orientation by clearly indicating workflow start and end points.

Proposed Implementation

Similar design as mermaid start and end points.

Start should be a circle similar to below

End should be a circle within a circle similar to below

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.