Add confirmation step to delete and clear drawing board

The drawing board can be deleted instantly by pressing the `Delete` key; if this is done by mistake it’s impossible to restore the table unless you undo immediately.

» the same is true for clearing the board with the trash icon; this cannot be undone even with undo

Possible solutions:

* add confirm delete step when delete key is pressed (same for trash icon?)
* record version history in whiteboard (similar to scribbles) to restore prev. version if needed

Start by locating the drawing-board actions triggered by the Delete key and trash icon. Clarify whether the intended outcome is a confirmation step, recoverable version history, or both; done means accidental deletion and clearing can be safely recovered according to the chosen behavior.

Written by the indexing model from the issue text.