feat: Add reusable error toast for failure feedback

[lornakelly]

Description

Introduce a shared toast mechanism from shadcn for surfacing errors (like download file and copy to clipboard failures). Anchor to bottom-right corner as top right would cover sidepanel badge etc

Motivation

Inline validation already covers node errors, but operational failures currently have nowhere to go - they're only logged to console where users never see them so dont know what went wrong. One reusable component means every failure path surfaces the same way.

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.