fix: prevent React StrictMode double execution in Verify

Added hasVerified flag to prevent useEffect from running twice
in React 18 StrictMode (development mode).

This was causing:
1. First call: 200 OK - verification successful
2. Second call: 400 Bad Request - already verified
3. Error shown to user despite successful verification

The fix ensures verify() only runs once per component mount.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/Verify.jsx

@@ -14,15 +14,17 @@ export default function Verify() {
   const [email, setEmail] = useState('')
   const [resending, setResending] = useState(false)
   const [resendSuccess, setResendSuccess] = useState(false)
+  const [hasVerified, setHasVerified] = useState(false)
 
   useEffect(() => {
+    if (hasVerified) return // Prevent React StrictMode double execution
+    if (!token) {
+      setStatus('error')
+      setError('Kein Verifikations-Token gefunden')
+      return
+    }
+    setHasVerified(true)
     const verify = async () => {
-      if (!token) {
-        setStatus('error')
-        setError('Kein Verifikations-Token gefunden')
-        return
-      }
-
       try {
         const result = await api.verifyEmail(token)