Artboard grab scale

[JRNukala]

Summary

Closes GraphiteEditor#4026
When the Artboard tool is selected, pressing G (grab) or S (scale) now triggers the same keyboard-driven transformation that the Select and Path tools already support. This reuses the existing GRS infrastructure inTransformLayerMessageHandler — no new transformation logic was added.

Changes

• transformation.rs: Include ToolType::Artboard in the OriginalTransforms::Layer gate so the GRS system snapshots artboard layer transforms. Also bypass the artboard filter in Selected::bounding_box() when the Artboard tool is active.
• pivot.rs: Thread active_tool_type through recalculate_pivot() and recalculate_transform() so the pivot includes artboards when the Artboard tool is active.
• select_tool.rs: Thread active_tool_type through create_bounding_box_transform() so GRS computes correct local axes for artboards. Existing Select tool call sites pass ToolType::Select to preserve current behavior.
• transform_layer_message_handler.rs: Pass the active tool type to the updated pivot and bounding-box functions. Block Rotate (R) for artboards since their bounds are axis-aligned integer rectangles.
• artboard_tool.rs: Add G/S keyboard hint to the Ready state so users can discover the shortcuts.

Test plan

• Select the Artboard tool, click an artboard, press G, move mouse, confirm with Enter — artboard should move
• Same flow with S — artboard should scale from the pivot
• Press Escape during G or S — artboard should revert to original position/size
• Press R with the Artboard tool — should be a no-op (no rotation)
• Verify that G/R/S on the Select tool and Path tool still work as before
• Check that the hint bar shows "G / S Grab/Scale Selected" in the Artboard tool's Ready state