[Grida Canvas] Better Copy & Paste positioning

[softmarshmallow]

Currently, copy-paste is purely based on the 'copied' items' transform.
When user wants to paste the copied items after panning (where the copied items origin transform is no longer in area) The editor should be smart enough to tell this and paste the item in the center of the current viewport. (even if there is not selected paste target)

Copy under selected target is implemented, this is the current alternative how user can do the "copy move" in a practical sense.

[k08200]

Added regression coverage in #699 for both scene-level paste and container-target paste. The current implementation already routes empty-selection paste to the scene target and applies the viewport-aware delta before parent adjustment, so the PR closes this with tests.