Fix new nodes being added in the wrong place near graph edges

[Keavon]

Fixes #4205

[gemini-code-assist]

Code Review

This pull request decouples the node creation coordinates from the context menu display coordinates. It introduces a new 'node_creation_coordinates' field to 'ContextMenuInformation' to store the original click point, ensuring that newly created nodes are placed at the exact click location even if the context menu itself is shifted to fit on-screen. The frontend 'Graph.svelte' has been updated to use these new coordinates when creating a node. I have no feedback to provide.

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[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

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

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