fix(buchhaltung): prevent form reset on query refetch during transaction edit

frontend/src/pages/BuchhaltungTransaktionForm.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useRef } from 'react';
 import { useParams, useNavigate, useSearchParams } from 'react-router-dom';
 import { useQuery, useMutation } from '@tanstack/react-query';
 import {
@@ -74,9 +74,12 @@ export default function BuchhaltungTransaktionForm() {
   // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [haushaltsjahre]);
 
-  // Pre-fill from existing transaction when editing
+  const hasPrefilled = useRef(false);
+
+  // Pre-fill from existing transaction when editing — only once
   useEffect(() => {
-    if (existing) {
+    if (existing && !hasPrefilled.current) {
+      hasPrefilled.current = true;
       setForm({
         haushaltsjahr_id: existing.haushaltsjahr_id,
         typ: existing.typ as 'einnahme' | 'ausgabe',