tablet: split price/pfand, light mode, bigger bold text

- show drink price and Pfand separately on tiles and in cart
- add light/dark toggle on the bar-picker page (persisted)
- theme via CSS variables, applied on load for tablet + admin
- larger, bolder text throughout; much bigger total sum

client/src/admin/main.tsx

@@ -1,4 +1,6 @@
 import { render } from 'preact';
 import { Admin } from './Admin';
+import { applyTheme } from '../api';
 
+applyTheme();
 render(<Admin />, document.getElementById('app')!);