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/components/PieceItem.vue

@@ -9,7 +9,7 @@
 
     <!-- Piece Header (collapsible, same pattern as ComponentItem) -->
     <div class="flex items-start justify-between p-4 bg-base-200 rounded-lg">
-      <div class="flex items-start gap-3 w-full">
+      <div class="flex items-start gap-3 flex-1 min-w-0">
         <button
           type="button"
           class="btn btn-ghost btn-sm btn-circle shrink-0 transition-transform"
@@ -21,7 +21,7 @@
           <IconLucideChevronRight class="w-5 h-5 transition-transform" aria-hidden="true" />
           <span class="sr-only">{{ isCollapsed ? 'Déplier' : 'Replier' }} la pièce</span>
         </button>
-        <div class="flex-1">
+        <div class="flex-1 min-w-0">
           <h3 class="text-lg font-semibold">
             {{ pieceData.name }}
           </h3>
@@ -49,6 +49,15 @@
           </div>
         </div>
       </div>
+      <button
+        v-if="showDelete"
+        type="button"
+        class="btn btn-ghost btn-xs text-error shrink-0"
+        title="Supprimer cette pièce"
+        @click="$emit('delete')"
+      >
+        Supprimer
+      </button>
     </div>
 
     <div v-show="!isCollapsed" class="space-y-4">
@@ -250,11 +259,12 @@ import { useEntityCustomFields } from '~/composables/useEntityCustomFields'
 const props = defineProps({
   piece: { type: Object, required: true },
   isEditMode: { type: Boolean, default: false },
+  showDelete: { type: Boolean, default: false },
   collapseAll: { type: Boolean, default: true },
   toggleToken: { type: Number, default: 0 },
 })
 
-const emit = defineEmits(['update', 'edit', 'custom-field-update'])
+const emit = defineEmits(['update', 'edit', 'custom-field-update', 'delete'])
 
 // --- Local reactive data for editing ---
 const pieceData = reactive({