Add freeform screenshot capture using a drawn selection #6

Problem

TinyHand can draw over the screen, but there is no mode for outlining content and capturing just the area inside that shape.

Proposed behavior

• Add an explicit Screenshot mode.
• Let the user trace a freeform shape; close the outline automatically and provide an action to save the selected area.
• Capture the desktop content inside the shape, crop to its bounds, and make the exterior transparent.
• Exclude the pointer, drawing overlays, selection outline, and toolbar from the image.
• Use a native PNG Save dialog and preserve the selection when saving is cancelled.
• Request Screen Recording permission only when capturing, and explain how to enable access if denied.

Proposed implementation

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