Added a clear first point distinction when drawing a custom shape: if the mouse cursor is near the beginning of a line (the shape can be closed), the first point is drawn in yellow and larger. This allows the user to see when they can close the shape with a single click. The code has been compiled and is ready to use. The functionality works as expected.
Introduces ShapeTool to allow users to define custom polygonal output areas by holding Shift+S and clicking to add points. The selected shape is used to crop and mask images and layers, and is visualized on the canvas. Updates Canvas, CanvasIO, CanvasInteractions, CanvasLayers, CanvasRenderer, and types to support shape-based output areas, including shape-aware import, export, and rendering logic.
