Default to dark mode outside 6:30-21:00 based on the visitor's clock

Only applies as the initial value when no theme preference is stored
yet - a manual toggle still sticks afterward, same precedent as the
browser-language auto-detection.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

hifi-src/src/context/ThemeContext.jsx

@@ -2,11 +2,20 @@ import { createContext, useContext, useEffect, useState } from 'react';
 
 const ThemeContext = createContext(null);
 
+// Hellmodus 6:30-21:00 Uhr, sonst Dunkelmodus - gilt nur als Startwert, solange noch
+// keine manuelle Wahl gespeichert ist (die manuelle Umschaltung bleibt danach bestehen,
+// genau wie bei der Spracherkennung).
+function detectDefaultTheme() {
+  const now = new Date();
+  const hour = now.getHours() + now.getMinutes() / 60;
+  return hour >= 6.5 && hour < 21 ? 'light' : 'dark';
+}
+
 export function ThemeProvider({ children }) {
   const [theme, setTheme] = useState(() => {
     const stored = localStorage.getItem('hifi-theme');
-    if (stored) return stored;
-    return 'light';
+    if (stored === 'light' || stored === 'dark') return stored;
+    return detectDefaultTheme();
   });
 
   useEffect(() => {