feat: Apply node styling (UX)

[lornakelly]

Description

Apply the agreed node design to workflow nodes. This includes implementing the per-task-type colour palette and any associated visual styling needed to align the nodes with the intended user experience

Motivation

Improve diagram clarity, accessibility and visual consistency by making node types easier to distinguish at a glance

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.