refactor(category) : remove quantity field from category structure editor

Quantity is now managed per-component on the component edit page,
not at the category level.

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

app/components/StructureNodeEditor.vue

@@ -258,18 +258,7 @@
                       {{ piece.typePieceId ? `Sélection : ${getPieceTypeLabel(piece.typePieceId) || 'Inconnue'}` : 'Aucune famille sélectionnée' }}
                     </p>
                   </div>
-                  <div class="form-control">
-                    <label class="label py-1"><span class="label-text text-xs">Quantité</span></label>
-                    <input
-                      v-model.number="piece.quantity"
-                      type="number"
-                      :min="1"
-                      step="1"
-                      placeholder="Qté"
-                      class="input input-bordered input-sm md:input-md w-20"
-                      @input="piece.quantity = Math.max(1, piece.quantity || 1)"
-                    />
-                  </div>
+                  <!-- Quantity is set per-component on the component edit page -->
                 </div>
                 <button type="button" class="btn btn-error btn-xs btn-square" @click="removePiece(index)">
                   <IconLucideTrash class="w-4 h-4" aria-hidden="true" />