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/api.ts

@@ -27,6 +27,22 @@ export function formatCents(cents: number): string {
   return `${sign}${(abs / 100).toFixed(2)} €`;
 }
 
+export type Theme = 'dark' | 'light';
+const THEME_KEY = 'wutz.theme';
+
+export function getTheme(): Theme {
+  return localStorage.getItem(THEME_KEY) === 'light' ? 'light' : 'dark';
+}
+
+export function applyTheme(t: Theme = getTheme()): void {
+  document.documentElement.setAttribute('data-theme', t);
+}
+
+export function setTheme(t: Theme): void {
+  localStorage.setItem(THEME_KEY, t);
+  applyTheme(t);
+}
+
 export function uuid(): string {
   if (typeof crypto !== 'undefined' && 'randomUUID' in crypto) {
     return (crypto as any).randomUUID();