Fix: doppelter useRef/useCallback Import entfernt

frontend/src/App.jsx

@@ -1,4 +1,4 @@
-import { useState, useEffect, useRef, useCallback, useRef, useCallback } from 'react';
+import { useState, useEffect, useRef, useCallback } from 'react';
 import { api, S } from './lib.js';
 import { TOOLS, getGroupedTools } from './toolRegistry.js';
 import CalendarWidget, { CalendarSettings } from './calendar.jsx';