refactor: create shared UI components and utilities

- Add Select UI component
- Create currency utilities (CURRENCIES, formatPrice)
- Create ThemedCard wrapper component
- Create FormField wrapper component
- Update ItemForm and WishlistItem to use new abstractions
- Standardize currency type with single source of truth

src/lib/components/ui/select/index.ts

@@ -0,0 +1,6 @@
+import Root from './select.svelte';
+
+export {
+  Root,
+  Root as Select
+};