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/pages/component/[id]/edit.vue

@@ -195,7 +195,7 @@
                 class="form-control"
               >
                 <label class="label">
-                  <span class="label-text text-xs font-medium">{{ slot.label }}</span>
+                  <span class="label-text text-xs font-medium" :class="{ 'text-error font-semibold': slot.isEmpty }">{{ slot.label }}</span>
                 </label>
                 <div class="flex items-start gap-2">
                   <div class="flex-1">
@@ -231,7 +231,7 @@
                 class="form-control"
               >
                 <label class="label">
-                  <span class="label-text text-xs font-medium">{{ slot.label }}</span>
+                  <span class="label-text text-xs font-medium" :class="{ 'text-error font-semibold': slot.isEmpty }">{{ slot.label }}</span>
                 </label>
                 <ProductSelect
                   :model-value="slot.selectedProductId"
@@ -252,7 +252,7 @@
                 class="form-control"
               >
                 <label class="label">
-                  <span class="label-text text-xs font-medium">{{ slot.label }}</span>
+                  <span class="label-text text-xs font-medium" :class="{ 'text-error font-semibold': slot.isEmpty }">{{ slot.label }}</span>
                 </label>
                 <ComposantSelect
                   :model-value="slot.selectedComponentId"