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/composables/useMachineHierarchy.ts

@@ -227,11 +227,13 @@ export const buildMachineHierarchyFromLinks = (
       const definition = (def.definition && typeof def.definition === 'object' ? def.definition : def) as AnyRecord
       const resolved = (def.resolvedPiece && typeof def.resolvedPiece === 'object' ? def.resolvedPiece : null) as AnyRecord | null
       const quantity = typeof definition.quantity === 'number' ? definition.quantity : (typeof def.quantity === 'number' ? def.quantity : 1)
+      const isEmpty = !resolved
+      const typePieceName = (resolved?.typePiece as AnyRecord)?.name || (definition.typePiece as AnyRecord)?.name || (def.typePiece as AnyRecord)?.name || null
       return {
         ...(resolved || {}),
         id: resolved?.id || `structure-piece-${composantId}-${index}`,
         pieceId: resolved?.id || null,
-        name: resolved?.name || definition.role || definition.name || def.role || def.name || `Pièce ${index + 1}`,
+        name: resolved?.name || definition.role || definition.name || def.role || def.name || (typePieceName ? `${typePieceName}` : `Pièce ${index + 1}`),
         reference: resolved?.reference || definition.reference || def.reference || null,
         prix: resolved?.prix ?? null,
         constructeurs: resolved?.constructeurs || [],
@@ -243,6 +245,7 @@ export const buildMachineHierarchyFromLinks = (
         parentComponentLinkId: machineComponentLinkId,
         parentComponentName: componentName,
         _structurePiece: true,
+        _emptySlot: isEmpty,
       }
     }) as AnyRecord[]