feat(ui): duplicate a plate by Ctrl+dragging it or from its right-click menu

Copying a layout meant recreating the plate and copying its parts by hand.
Ctrl+drag a row in the Plates list drops a copy at that slot; the row's
right-click menu has Duplicate Plate, which inserts the copy after it.

Plate.CopyLayout copies the sheet (size, outline, spacing, quadrant, grain,
clamps, support bed), the parts with their programs and lead-in state,
common-line groups and cut-off definitions, regenerated in their places in
the sequence, as a save and reload would. The copy is one sheet with no
stock item codes, since those name particular physical sheets. The desktop
gives it its own cutting parameters and records it as one undo step.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/undo-redo.md

@@ -21,7 +21,8 @@ Everything stored in the nest:
 
 - parts: add, delete, cut, move, drag, nudge, rotate, align, push, center, expand spacing, clone;
 - placement: fill, fill area, pattern tile, best-fit placement, Auto Nest, Clear Nest;
-- plates: add, remove, remove empty plates, Move Plate (dragging a row in the Plates list or
+- plates: add, remove, remove empty plates, Duplicate Plate (Ctrl+dragging a row in the Plates
+  list or its right-click menu), Move Plate (dragging a row in the Plates list or
   Alt+Up/Alt+Down), Edit Plate (its sheet outline included), rotate,
   resize, cutting parameters, clamps and clamp rail, the support grid, the nest's default plate;
 - cutting: manual and automatic cut-offs and their drags, Auto Sequence, Set Sequence,