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/tablet/BarPicker.tsx

@@ -1,4 +1,6 @@
+import { useState } from 'preact/hooks';
 import type { Bar } from '@wutzcalc/shared';
+import { getTheme, setTheme } from '../api';
 
 interface Props {
   bars: Bar[] | null;
@@ -6,6 +8,14 @@ interface Props {
 }
 
 export function BarPicker({ bars, onPick }: Props) {
+  const [theme, setThemeState] = useState(getTheme());
+
+  function toggleTheme() {
+    const next = theme === 'dark' ? 'light' : 'dark';
+    setTheme(next);
+    setThemeState(next);
+  }
+
   return (
     <div class="bar-picker">
       <h1>Bar auswählen</h1>
@@ -18,6 +28,9 @@ export function BarPicker({ bars, onPick }: Props) {
           </button>
         ))
       )}
+      <button class="theme-toggle" onClick={toggleTheme}>
+        {theme === 'dark' ? '☀ Heller Modus' : '🌙 Dunkler Modus'}
+      </button>
     </div>
   );
 }