feat(borç): aylık planda bu ayın borcu, borç ödemesi etiketi, taksit önerisi

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/debt.ts

@@ -159,6 +159,24 @@ export function useSetTargets() {
   })
 }
 
+export function useSetOpeningDebt(periodId: string) {
+  const invalidate = useInvalidateDebt()
+  return useMutation({
+    mutationFn: (amount: number | null) =>
+      api.put<void>(`/periods/${periodId}/opening-debt`, { amount }),
+    onSuccess: invalidate,
+  })
+}
+
+/** Plan kalemlerinde "Taksit(ler)" adlı sabit gider (büyük/küçük harf ve Türkçe harf duyarsız) */
+export function findInstallmentItem<T extends { kind: string; name: string }>(
+  items: readonly T[],
+): T | undefined {
+  return items.find(
+    (i) => i.kind === 'expense' && /^taks[iı]t(ler|lar)?$/.test(i.name.trim().toLocaleLowerCase('tr')),
+  )
+}
+
 export function useDebtImport() {
   const invalidate = useInvalidateDebt()
   return useMutation({