Prep and Capacity: Undo last corner while drawing a requirement area

Suite 2.246.14, Prep 2.10.0, Capacity 1.13.0.

What changed

A new "Undo last corner" button takes back the last corner clicked while drawing a shape. It sits between "Draw a shape" and "Back to automatic" in the "Draw the area" panel. Each press removes one corner, so a mis-click partway through a long outline no longer means starting again. The button is greyed out until a shape is being drawn and at least one corner has been placed; taking every corner back leaves "Draw a shape" active so drawing can continue.
Ctrl+Z (Cmd+Z on a Mac) does the same. Backspace and Delete continue to work as before.
The line under the buttons counts the corners placed while a shape is being drawn, for example "Click each corner (5 so far)", and names the three ways to take one back.
A corner placed again on the spot just taken back is now treated as a new corner. Previously a click on the same spot straight after an undo could be read as a double-click and finish the shape early.
"Draw a rectangle" is unchanged: a rectangle is one drag, and Esc still cancels it.

Tests drive the editor with real mouse and keyboard events on a real plan view and check the corners that arrive: one corner removed per press, nothing removed when none are placed or when no shape is being drawn, Ctrl+Z and Cmd+Z undo while Shift+Ctrl+Z does not, a corner clicked after an undo lands where it was clicked, and the buttons and the corner count follow each click and each undo. The button's handler is taken from the rendered markup and run. Each check fails when the matching behaviour is removed.
The full suite passes.