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/MachineDocumentsCard.vue

@@ -74,7 +74,7 @@
             <button
               v-if="isEditMode"
               type="button"
-              class="btn btn-error btn-xs"
+              class="btn btn-ghost btn-xs text-error"
               :disabled="uploading"
               @click="$emit('remove', doc.id)"
             >