Support both arbitrary free selection and edge-dragging for cutting region

[Hasanhasan323]

In addition to allowing the user to resize the default area by dragging its edges, Gradia should also let the user manually specify the cut region (e.g., by clicking to define new vertices or boundaries).

Video_2026-07-02_23-08-20.mov

For example like GIMP:

Video_2026-07-02_23-09-02.mov