Add G/S shortcut handling to Artboard tool

[devviktoria]

Adds support G/S (grab and scale) keyboard shortcut handling to the Artboard tool.

Closes #4026

[gemini-code-assist]

Code Review

This pull request implements keyboard shortcuts for grabbing and scaling artboards within the Artboard tool by adding a new GS message and updating the FSM logic. Review feedback highlights that the scaling implementation is incomplete, lacking necessary state initializations and document transactions required for the ResizingBounds state to function correctly. Additionally, it is recommended to remove commented-out code in the message handler.

[gemini-code-assist]

The scale branch in the GS message handler is missing several critical initializations required for the ResizingBounds state to function correctly. Specifically, it needs to:

1. Start a document transaction so the operation can be undone/aborted.
2. Initialize bounding_box_manager.selected_edges and opposite_pivot. Without selected_edges, the resize_artboard function (called during PointerMove) will return early and do nothing.
3. Call tool_data.start_resizing to set up the center of transformation and initial location.

Since this is a keyboard shortcut, you might want to default to resizing from the bottom-right corner or similar.

[gemini-code-assist]

Please remove the commented-out code. If the functionality is intended to be part of this PR, it should be implemented; otherwise, it should be removed to keep the codebase clean.