Add Ctrl+Enter/Esc and drag-handle reorder to LET to LAMBDA editor

[jimmytacks]

Summary

• Ctrl+Enter saves (when validation passes); Esc cancels — both via a window-level PreviewKeyDown handler in LetToLambdaWindow.
• The ▲/▼ move buttons on each input row are replaced with a drag handle (⋮⋮) on the left of each kept row.
• Drag/drop is minimal and in-window: source must be a kept row, drop target must be a kept row. Unchecked rows refuse the drop, so the existing kept-rows-first invariant is preserved without additional bookkeeping.
• Alt+Up / Alt+Down keyboard reorder still works (and is now the only Move… code path the row template references, alongside drag/drop and the existing Keep-toggle reposition).

Took the custom approach (~80 lines) rather than adding gong-wpf-dragdrop as a dependency — happy to switch if the UX falls short.

Test plan

Unit tests pass (433 / 433). Drag/drop and the new shortcuts are WPF UI behaviour that needs Excel + a real STA window, so manual checks:

• Open Edit Lambda or LET to LAMBDA and confirm the row template renders the new drag handle on the left and no ▲/▼ buttons on the right
• Drag a kept row up/down by the handle; row reorders, the rest of the list reflows
• Drag onto an unchecked row → cursor shows "no-drop"; drop is rejected
• Uncheck a row → its handle disappears and it can no longer be dragged
• Alt+Up / Alt+Down still reorders the focused kept row
• Ctrl+Enter saves only when the Save button is enabled (try with an empty/invalid Lambda name → no-op; with a valid name → saves and closes)
• Esc closes the dialog with no result

Closes #101

🤖 Generated with Claude Code

[jimmytacks]

Switched to gong-wpf-dragdrop (3.2.x) per discussion — the library handles the insertion-line adorner and drag-preview ghost natively, which were the missing UX in the custom implementation.

Changes in 90c5698:

• Added gong-wpf-dragdrop package reference
• InputsList gets dd:DragDrop.IsDragSource / IsDropTarget / UseDefaultDragAdorner
• TextBox and CheckBoxes marked dd:DragDrop.DragSourceIgnore so text selection / clicks still work
• New KeptRowReorderHandler (single class implementing both IDragSource and extending DefaultDropHandler) enforces the kept-rows-only rule on both ends; DefaultDropHandler.Drop does the ObservableCollection.Move
• All custom mouse-handler / DragOver / Drop code is gone (~100 fewer lines)
• _rows.CollectionChanged hook now drives UpdateSaveEnabled after a Move

Net diff: −95 lines, +72 lines including the handler class and a single new import. Build clean, 433/433 unit tests pass. Same manual test plan as the original PR description.