feat(ui) : add site colors, dark mode toggle and card styling improvements

- Site color field with color picker in create/edit modals
- Dark mode theme (mytheme-dark) with toggle in navbar
- Stronger site color visibility on cards (gradient, top border, badges)
- Bigger action buttons (btn-sm) on machine cards
- White card backgrounds with proper dark mode support

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

app/pages/constructeurs.vue

@@ -49,11 +49,11 @@
           </template>
 
           <template #cell-actions="{ row }">
-            <div class="flex justify-end gap-2">
+            <div class="flex items-center justify-end gap-2">
               <button class="btn btn-ghost btn-xs" @click="openEditModal(row)">
                 {{ canEdit ? 'Modifier' : 'Consulter' }}
               </button>
-              <button v-if="canEdit" class="btn btn-error btn-xs" @click="confirmDelete(row)">
+              <button v-if="canEdit" class="btn btn-ghost btn-xs text-error" @click="confirmDelete(row)">
                 Supprimer
               </button>
             </div>