Add layer and artboard nudge resizing

[Keavon]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces the ability to resize selected layers and artboards using arrow-key nudges combined with modifier keys (Alt to resize, and Control to resize from the opposite corner). It updates the input mappings, message definitions, and handlers across the artboard, shape, and select tools, and implements a new utility function nudge_resize_bounds to calculate the updated bounds and scale transformations. Since there are no review comments, I have no additional feedback to provide.

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[cubic-dev-ai]

1 issue found and verified against the latest diff

Confidence score: 3/5

• In editor/src/messages/portfolio/document/document_message_handler.rs, the resize path’s combined bounding box can be poisoned by a single non-finite layer bound, which can cascade into invalid scale values and broken resize behavior for users — add a finite/NaN guard (skip or sanitize non-finite bounds) before computing the combined box before merging.

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