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/MachineCustomFieldDefEditor.vue

@@ -84,7 +84,7 @@
 
           <button
             type="button"
-            class="btn btn-error btn-xs btn-square"
+            class="btn btn-ghost btn-xs btn-square text-error"
             @click="$emit('remove-field', index)"
           >
             <IconLucideTrash class="w-4 h-4" aria-hidden="true" />