Ctrl+Z should undo and Ctrl+Shift+Z should redo, on Windows and Linuxes.

If you're worried about conflicting with native browser undo/redo, well, you can defer to the default behavior if any text field is focused and preventDefault() otherwise.

If you're concerned with consistency between shortcuts, with Ctrl+N being unusable in some (maybe all or most) browsers, you could support both Ctrl and Alt+[Shift+]Z.

Also, I would support Ctrl+Y for redo, as many Windows users are used to.