Guide sketch patterns with live previews and editable offsets (#966)

Sketch Mirror, rectangular and circular Pattern, and Offset now show a green live preview that follows the selection and settings; OK commits, Cancel leaves the sketch unchanged. The shared dialogs show the current selection step and source counts, sources can be toggled with Ctrl/Cmd-click, cleared and reselected.

Offsets keep editable driving distance dimensions (One Side, Two Sides, Symmetric) that update the derived curves without moving the source, with undo/redo and save/reopen. Format 19: older files keep their fixed offsets.

New dialog labels in en, zh, ja, de and ko.

Fixes #696. Fixes #517.

crates/engine/src/cmd/sketch_constraints.rs

@@ -430,6 +430,7 @@ fn anchor(sk: &Sketch, k: &ConstraintKind) -> Option<Vec2> {
     };
     let pt = |p: usize| sk.point(p);
     match *k {
+        OffsetDistance { ref output, .. } => sk.offset_dimension_points(output).map(|(a, b)| (a + b) * 0.5),
         Coincident { p, .. } | Fix { p } | Midpoint { p, .. } => pt(p),
         PointOnCurve { p, .. } => pt(p),
         HorizontalPoints { p, q } | VerticalPoints { p, q } | Symmetric { p, q, .. } => Some((pt(p)? + pt(q)?) * 0.5),