Implement dragging layers into the group/new/delete buttons in the Layers panel

[Keavon]

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[cubic-dev-ai]

No issues found across 8 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[gemini-code-assist]

Code Review

This pull request implements a drag-and-drop system for the Layers panel, allowing users to drop layers onto action buttons such as Group, New Layer, and Trash. The changes include a new WidgetValueDragDrop message in the backend, an on_drag_drop callback for IconButton widgets, and frontend logic in Svelte to handle drag detection and event dispatching. Review feedback recommends refactoring the backend message handler to use existing callback logic for better maintainability and adopting more idiomatic Svelte event directives in the IconButton component.