Point the motion imports at the renamed package

Renovate's replacement PR swapped framer-motion for its successor 'motion' in
package.json but left the imports, so the build could not resolve them. The React entry
point is motion/react.

frontend/src/components/Header.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react';
-import { motion, AnimatePresence } from 'framer-motion';
+import { motion, AnimatePresence } from 'motion/react';
 import { Link } from 'react-router-dom';
 import Logo from './Logo';