feat(BillingManager): implement next payment date calculation based on payment history and exchange rates

frontend/src/BillingManager.jsx

@@ -117,6 +117,41 @@ function BillingManager() {
     fetchServers();
   }, []);
 
+  // Пересчёт nextPaymentDate по платежам при загрузке (для уже сохранённых данных)
+  useEffect(() => {
+    if (billingData.length === 0 || ratesLoading) return;
+    setBillingData(prev => {
+      const updated = prev.map(item => {
+        const payments = item.payments || [];
+        if (payments.length === 0) return item;
+        const last = payments.slice().sort((a, b) => new Date(b.date) - new Date(a.date))[0];
+        const computed = (() => {
+          if (!last?.date) return null;
+          const amountRUB = (amt, ccy) => {
+            if (!amt) return 0;
+            const rub = Number(exchangeRates.RUB) || 1;
+            if (!ccy || ccy === 'RUB') return amt;
+            if (ccy === 'USD') return amt * rub;
+            const c = Number(exchangeRates[ccy]);
+            return c ? (amt / c) * rub : amt * rub;
+          };
+          const costRUB = amountRUB(item.monthlyCost || 0, item.monthlyCostCurrency || 'USD');
+          const amtRUB = amountRUB(last.amount, last.currency || 'USD');
+          const months = costRUB > 0 ? Math.max(1, Math.floor(amtRUB / costRUB)) : 1;
+          const d = new Date(last.date);
+          d.setMonth(d.getMonth() + months);
+          return d.toISOString().slice(0, 10);
+        })();
+        const current = new Date(item.nextPaymentDate || 0);
+        const now = new Date();
+        if (computed && current < now) return { ...item, nextPaymentDate: computed };
+        return item;
+      });
+      const changed = updated.some((it, i) => it.nextPaymentDate !== prev[i]?.nextPaymentDate);
+      return changed ? updated : prev;
+    });
+  }, [billingData.length, ratesLoading, exchangeRates]);
+
   // Автоматическая синхронизация: добавляем серверы без записей в биллинге
   useEffect(() => {
     if (servers.length === 0 || loading) return;
@@ -295,11 +330,14 @@ function BillingManager() {
     return Math.ceil((nextPayment - now) / (1000 * 60 * 60 * 24));
   };
 
-  // Добавить 1 месяц к дате (для обновления nextPaymentDate после платежа)
-  const addMonthToDate = (dateString) => {
-    if (!dateString) return '';
-    const d = new Date(dateString);
-    d.setMonth(d.getMonth() + 1);
+  // Вычислить nextPaymentDate по платежу: сумма / месячная стоимость = кол-во месяцев
+  const computeNextPaymentDate = (payment, server) => {
+    if (!payment?.date) return '';
+    const amountRUB = convertToRUB(payment.amount, payment.currency || 'USD');
+    const costRUB = convertToRUB(server.monthlyCost || 0, server.monthlyCostCurrency || 'USD');
+    const months = costRUB > 0 ? Math.max(1, Math.floor(amountRUB / costRUB)) : 1;
+    const d = new Date(payment.date);
+    d.setMonth(d.getMonth() + months);
     return d.toISOString().slice(0, 10);
   };
 
@@ -467,7 +505,7 @@ function BillingManager() {
       const payments = [...(s.payments || [])];
       payments.splice(index, 1);
       const last = payments.slice().sort((a,b) => new Date(b.date) - new Date(a.date))[0];
-      const nextPaymentDate = addMonthToDate(last?.date);
+      const nextPaymentDate = last ? computeNextPaymentDate(last, s) : '';
       return {
         ...s,
         payments,
@@ -1091,7 +1129,7 @@ function BillingManager() {
                 note: paymentDraft.note || ''
               };
               const last = payments.slice().sort((a,b) => new Date(b.date) - new Date(a.date))[0];
-              const nextPaymentDate = addMonthToDate(last?.date);
+              const nextPaymentDate = computeNextPaymentDate(last, s);
               return { ...s, payments, lastPaymentDate: last?.date || '', lastPaymentAmount: last?.amount || 0, lastPaymentCurrency: last?.currency || 'USD', nextPaymentDate: nextPaymentDate || s.nextPaymentDate };
             }));
             setEditingPayment(null);
@@ -1105,7 +1143,7 @@ function BillingManager() {
                 note: paymentDraft.note || '' 
               }];
               const last = payments.slice().sort((a,b) => new Date(b.date) - new Date(a.date))[0];
-              const nextPaymentDate = addMonthToDate(last?.date);
+              const nextPaymentDate = computeNextPaymentDate(last, s);
               return { ...s, payments, lastPaymentDate: last?.date || '', lastPaymentAmount: last?.amount || 0, lastPaymentCurrency: last?.currency || 'USD', nextPaymentDate: nextPaymentDate || s.nextPaymentDate };
             }));
           }