feat: Implement drag-and-drop YAML/JSON workflow loading in Storybook

[lornakelly]

Description

Add support for dragging and dropping a workflow YAML or JSON file onto the Storybook canvas and rendering it in the editor. This is intended as a development and debugging aid, making it easier to quickly test and experiment with different workflow definitions without creating a dedicated story for each one.

With Storybook deployments available via PR previews, this will also allow others (e.g. reviewers or contributors) to drag and drop their own workflow files directly into the editor to test behaviour without needing a local setup.

Motivation

Improve the development workflow by turning Storybook into a lightweight sandbox for testing workflow content on demand, and make it easier for others to validate changes during PR reviews.

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.