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

app/api/billing/balance/route.js

@@ -0,0 +1,14 @@
+import { NextResponse } from 'next/server';
+import { requireUser } from '@/lib/session';
+import { engine } from '@/lib/engine';
+
+export async function GET(req) {
+  const user = await requireUser();
+  if (!user) return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  try {
+    const data = await engine.getBillingBalance(user.id);
+    return NextResponse.json(data);
+  } catch (err) {
+    return NextResponse.json({ error: err.message }, { status: 500 });
+  }
+}