feat: Apply container node styling (UX)

[lornakelly]

Description

Apply the agreed node design to workflow containment nodes for do, for, fork, and try. his includes styling the wrapper nodes so they are visually distinct while still clearly framing the child nodes they contain.

Motivation

Improve the readability of composite workflow structures by making container nodes easier to identify and distinguish from standard task nodes

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.