Add connected shape

Show plus buttons around a selected shape that create a new connected shape, like in Figma.

---

When a single shape is selected, four plus buttons appear on the edges of its selection bounds. Clicking one duplicates the shape in that direction and connects the two shapes with a bound arrow, then selects the new shape so you can keep extending the diagram.

The buttons are rendered in the InFrontOfTheCanvas component slot and positioned by converting the shape's page bounds to viewport coordinates with pageToViewport. The connection is made by creating an arrow shape and binding both of its terminals with createBindings.