Add Alt+drag layer duplication to the Layers panel

[Keavon]

Closes #2824.

[gemini-code-assist]

Code Review

This pull request introduces the ability to duplicate selected layers by holding the Alt key while dragging and dropping them in the layers panel. It adds a new DuplicateSelectedLayersTo message, implements a helper method duplicate_layer to handle copying a layer and its upstream node chain, and updates the frontend layers component to trigger duplication when the Alt key is pressed during a drag-and-drop operation. There are no review comments, so I have no feedback to provide.

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[cubic-dev-ai]

2 issues found across 4 files

Confidence score: 2/5

• There is a high-risk stability concern in editor/src/messages/portfolio/document/document_message_handler.rs: duplicate_layer() uses expect(...) on a runtime lookup, so a missing NodeId(0) can panic and crash the editor instead of failing gracefully.
• There is also a concrete user-facing regression risk in editor/src/messages/portfolio/document/document_message_handler.rs: DuplicateSelectedLayersTo reparents duplicates without preserving world-space transform, which can cause duplicated layers to visibly jump under transformed parents.
• Given the 6–7/10 severity issues and strong confidence on the panic path, this is likely not safe to merge until these behaviors are handled defensively.
• Pay close attention to editor/src/messages/portfolio/document/document_message_handler.rs - prevent panic-based failure in duplication and preserve world-space transforms when reparenting duplicates.

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

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

_{Re-trigger cubic}