feat(ui) : highlight empty slots with category name in red

- Empty component slots (pieces, products, subcomponents) now display
  the category/type name with red styling instead of generic labels
- Machine view: empty structure pieces show type name + "manquant" in red
- Backend: include typePiece in structure slot data for name resolution

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Controller/MachineStructureController.php

@@ -728,6 +728,7 @@ class MachineStructureController extends AbstractController
             $pieceData = [
                 'slotId'          => $slot->getId(),
                 'typePieceId'     => $slot->getTypePiece()?->getId(),
+                'typePiece'       => $this->normalizeModelType($slot->getTypePiece()),
                 'quantity'        => $slot->getQuantity(),
                 'selectedPieceId' => $slot->getSelectedPiece()?->getId(),
             ];