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/piece/[id].vue

@@ -224,7 +224,7 @@
                 class="form-control"
               >
                 <label class="label">
-                  <span class="label-text text-xs font-medium">
+                  <span class="label-text text-xs font-medium" :class="{ 'text-error font-semibold': !productSelections[entry.index] }">
                     {{ entry.label }}
                   </span>
                 </label>
@@ -244,10 +244,11 @@
                 class="form-control"
               >
                 <label class="label">
-                  <span class="label-text text-xs font-medium">{{ entry.label }}</span>
+                  <span class="label-text text-xs font-medium" :class="{ 'text-error font-semibold': !productSelectionLabels[index] }">{{ entry.label }}</span>
                 </label>
-                <div class="input input-bordered input-sm md:input-md bg-base-200 flex items-center">
-                  {{ productSelectionLabels[index] || '— Non sélectionné' }}
+                <div class="input input-bordered input-sm md:input-md flex items-center" :class="productSelectionLabels[index] ? 'bg-base-200' : 'border-error bg-error/10 text-error font-semibold'">
+                  <template v-if="!productSelectionLabels[index]">{{ entry.label }} — manquant</template>
+                  <template v-else>{{ productSelectionLabels[index] }}</template>
                 </div>
               </div>
             </div>