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/machine/MachinePiecesCard.vue

@@ -25,22 +25,15 @@
       </div>
 
       <div v-else class="space-y-2">
-        <div v-for="piece in pieces" :key="piece.id" class="relative">
-          <button
-            v-if="isEditMode"
-            type="button"
-            class="btn btn-error btn-xs absolute top-2 right-2 z-10"
-            title="Supprimer cette pièce"
-            @click="$emit('remove-piece', piece.linkId || piece.id)"
-          >
-            Supprimer
-          </button>
+        <div v-for="piece in pieces" :key="piece.id">
           <PieceItem
             :piece="piece"
             :is-edit-mode="false"
+            :show-delete="isEditMode"
             :collapse-all="collapsed"
             :toggle-token="collapseToggleToken"
             @edit="$emit('edit-piece', $event)"
+            @delete="$emit('remove-piece', piece.linkId || piece.id)"
           />
         </div>
       </div>