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>

frontend/app/components/PieceItem.vue

@@ -14,7 +14,7 @@
     />
 
     <!-- Piece Header (collapsible, same pattern as ComponentItem) -->
-    <div class="flex items-start justify-between p-4 bg-base-200 rounded-lg">
+    <div class="flex items-start justify-between p-4 rounded-lg" :class="piece._emptySlot ? 'bg-error/10 border border-error' : 'bg-base-200'">
       <div class="flex items-start gap-3 flex-1 min-w-0">
         <button
           type="button"
@@ -28,8 +28,9 @@
           <span class="sr-only">{{ isCollapsed ? 'Déplier' : 'Replier' }} la pièce</span>
         </button>
         <div class="flex-1 min-w-0">
-          <h3 class="text-lg font-semibold">
+          <h3 class="text-lg font-semibold" :class="{ 'text-error': piece._emptySlot }">
             {{ pieceData.name }}
+            <span v-if="piece._emptySlot" class="text-sm font-semibold text-error ml-1">— manquant</span>
             <span
               v-if="displayQuantity > 1"
               class="text-sm font-normal text-base-content/60 ml-1"