feat: billing UI — balance in header + /billing transactions page

- Header: Coins badge с кредитами, ссылка на /billing
- app/billing/page.js: баланс, план, стоимость операций, история транзакций
- app/api/billing/balance/route.js, transactions/route.js — прокси к engine
- lib/engine.js: getBillingBalance, getTransactions, getBillingPlans, adminCreditUser

components/Header.js

@@ -1,11 +1,22 @@
 'use client';
 import Link from 'next/link';
 import { useRouter } from 'next/navigation';
-import { Sparkles, LogOut, Settings2, CalendarDays, TrendingUp } from 'lucide-react';
+import { useEffect, useState } from 'react';
+import { Sparkles, LogOut, Settings2, CalendarDays, TrendingUp, Coins } from 'lucide-react';
 import ThemeToggle from './ThemeToggle';
 
 export default function Header({ user }) {
   const router = useRouter();
+  const [credits, setCredits] = useState(null);
+
+  useEffect(() => {
+    if (!user) return;
+    fetch('/api/billing/balance')
+      .then(r => r.json())
+      .then(d => setCredits(d.isUnlimited ? '∞' : d.credits))
+      .catch(() => {});
+  }, [user?.id]);
+
   async function logout() {
     await fetch('/api/auth/logout', { method: 'POST' });
     router.push('/login');
@@ -30,12 +41,15 @@ export default function Header({ user }) {
           )}
         </nav>
         <div className="flex items-center gap-2">
+          {/* Баланс кредитов */}
+          {credits !== null && (
+            <Link href="/billing" className="flex items-center gap-1.5 px-2.5 py-1 rounded-lg bg-accent/10 hover:bg-accent/20 transition-colors text-sm font-medium text-accent">
+              <Coins className="w-3.5 h-3.5" />
+              <span>{credits} кр</span>
+            </Link>
+          )}
           {user?.isAdmin && (
-            <Link
-              href="/system"
-              className="btn-ghost p-2 text-sm hidden sm:inline-flex"
-              title="Системные настройки"
-            >
+            <Link href="/system" className="btn-ghost p-2 text-sm hidden sm:inline-flex" title="Системные настройки">
               <Settings2 className="w-4 h-4" />
               <span className="hidden md:inline">Система</span>
             </Link>