fix(layer-editor): use stable _key for drag-and-drop sorting

Add a unique `_key` to each layer config to maintain stable sortable IDs
across reorder operations. Update LayerRow to use the sortableId prop
and the `use:sortable` directive. Refactor layer-crud utilities to
generate and propagate `_key` values on creation and initialization.

src/components/md-deck/hooks/deckStore.ts

@@ -463,8 +463,12 @@ export function createDeckStore(initialSrc: string = ""): DeckStore {
       setState({
         cards: data,
         activeTab: 0,
-        frontLayerConfigs: initLayerConfigsForSide(data, layersStr),
-        backLayerConfigs: initLayerConfigsForSide(data, backLayersStr),
+        frontLayerConfigs: layerCrud.withKeys(
+          initLayerConfigsForSide(data, layersStr),
+        ),
+        backLayerConfigs: layerCrud.withKeys(
+          initLayerConfigsForSide(data, backLayersStr),
+        ),
         isLoading: false,
       });
       updateDimensions();