Allow saving drawings as transparent PNG images #7

Problem

Screen drawings are currently temporary overlays with no option to save them. Users should be able to keep a sketch without taking a screenshot of the whole desktop.

Proposed behavior

• Add a Save Drawing action to the toolbar and menu.
• Export completed strokes as a PNG cropped to the artwork, with a transparent background and no desktop or pointer.
• Keep faded strokes available for export until the drawing is explicitly cleared or the pointer is turned off.
• Use a native Save dialog for the filename and destination; cancelling must preserve the drawing.

Proposed implementation

#5 implements this alongside native pointer interaction and freeform screenshots. It includes screenshot and video evidence and is open, not merged.