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/composables/useSites.ts

@@ -6,6 +6,7 @@ import { extractCollection } from '~/shared/utils/apiHelpers'
 export interface Site {
   id: string
   name?: string
+  color?: string
   contactName?: string
   contactPhone?: string
   contactAddress?: string