fix(ui) : move add buttons below last element in structure editors

Place "Ajouter" buttons after the items list instead of in the section
header, so they always appear below the last added element.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/components/machine/MachinePiecesCard.vue

@@ -3,31 +3,21 @@
     <div class="card-body">
       <div class="flex justify-between items-center mb-4">
         <h2 class="card-title">Pièces de la machine</h2>
-        <div class="flex items-center gap-2">
-          <button
-            v-if="isEditMode"
-            type="button"
-            class="btn btn-sm md:btn-md btn-primary"
-            @click="$emit('add-piece')"
-          >
-            Ajouter une pièce
-          </button>
-          <button
-            type="button"
-            class="btn btn-ghost btn-sm gap-2"
-            @click="$emit('toggle-collapse')"
-            :title="collapsed ? 'Déplier toutes les pièces' : 'Replier toutes les pièces'"
-          >
-            <IconLucideChevronRight
-              class="w-5 h-5 transition-transform"
-              :class="collapsed ? 'rotate-0' : 'rotate-90'"
-              aria-hidden="true"
-            />
-            <span class="text-sm">
-              {{ collapsed ? 'Tout déplier' : 'Tout replier' }}
-            </span>
-          </button>
-        </div>
+        <button
+          type="button"
+          class="btn btn-ghost btn-sm gap-2"
+          @click="$emit('toggle-collapse')"
+          :title="collapsed ? 'Déplier toutes les pièces' : 'Replier toutes les pièces'"
+        >
+          <IconLucideChevronRight
+            class="w-5 h-5 transition-transform"
+            :class="collapsed ? 'rotate-0' : 'rotate-90'"
+            aria-hidden="true"
+          />
+          <span class="text-sm">
+            {{ collapsed ? 'Tout déplier' : 'Tout replier' }}
+          </span>
+        </button>
       </div>
 
       <div v-if="pieces.length === 0" class="text-sm text-gray-500 py-4">
@@ -54,6 +44,15 @@
           />
         </div>
       </div>
+
+      <button
+        v-if="isEditMode"
+        type="button"
+        class="btn btn-sm md:btn-md btn-primary"
+        @click="$emit('add-piece')"
+      >
+        Ajouter une pièce
+      </button>
     </div>
   </div>
 </template>