Edit: Undo during typing or a drag takes back only that, and Redo returns it

edit.undo cancelled an open interaction (rolling back the typing session
or drag preview without recording it) and then undid the step before it:
one Undo removed two steps, and the typing could not be redone. Undo now
commits the interaction first, so it is the step taken back and Redo can
restore it; commit_interaction does nothing when nothing is open or
nothing changed.

Fixes #159

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U9TrG4w49oSRYbP3cqxA3D

crates/engine/src/cmd/edit.rs

@@ -117,7 +117,9 @@ fn set_units(s: &mut Session, p: &Value) -> Result<Value> {
 }
 
 fn undo(s: &mut Session, _: &Value) -> Result<Value> {
-    s.cancel_interaction()?;
+    // What is still in progress (typing, a drag) is the step to undo: keep it first, so Undo takes
+    // back only that and Redo brings it back, instead of dropping it and undoing the step before.
+    s.commit_interaction()?;
     let st = s.doc_mut()?;
     let e = st.history.undo.pop().ok_or_else(|| EngineError::Other("nothing to undo".into()))?;
     let label = e.label.clone();