feat(marketplace): Buy with sats for fixed-price listings

A price_type=fixed listing now shows a direct 'Buy with sats' button in
ListingTradeActions (card + detail) - no negotiation needed. The price resolves
to sats (SATS directly, USD via useUsdToSats) and pays through PayWithSats (NWC ->
WebLN -> Cashu melt, falling back to the seller's lud16). The sheet mounts only
while open so the wallet hooks stay off every grid card. Reuses the accepted-offer
Pay rail.

AGENTS.md

@@ -119,6 +119,14 @@ session with **no other memory of this project**. So:
     NIP.md). `creq`/`clink` are recognized but **not payable yet**. This is the
     **Numo/BTCPay-derived** "pay the seller in-app" rail; the request format is
     ours (offer tags), not a new kind.
+  - **Buy with sats — fixed price, no negotiation (2026-10-01):** a
+    `price_type=fixed` listing shows a direct **Buy with sats** button in
+    `ListingTradeActions` (shared by card + detail). The price is resolved to
+    sats — **SATS** directly, **USD** via `useUsdToSats` (live rate); other
+    currencies fall back to the offer/contact flow — and paid through the same
+    `PayWithSats` sheet (NWC → WebLN → Cashu melt, falling back to the seller's
+    `lud16`). ⚠️ The sheet is **mounted only while open** so its wallet hooks
+    don't run on every grid card.
 - **Cashu escrow** (marketplace): time-locked **P2PK** (lock to seller + buyer
   refund key + locktime) — see **`ESCROW.md`** and `src/lib/cashuEscrow.ts`
   (unit-tested). Protocol: `cashu_token` on the `31872`; each user advertises a

src/components/ListingTradeActions.tsx

@@ -14,8 +14,10 @@ import {
 import { useCashuEscrow } from '@/hooks/wallet/useCashuEscrow';
 import { EscrowRequestDialog } from '@/components/EscrowRequestDialog';
 import { EscrowPanel } from '@/components/EscrowPanel';
+import { PayWithSats } from '@/components/PayWithSats';
+import { useUsdToSats } from '@/hooks/useExchangeRate';
 import { parseEscrowToken } from '@/lib/cashuEscrow';
-import { MessageCircle, Loader2, CheckCircle, Lock, Send, RotateCcw } from 'lucide-react';
+import { MessageCircle, Loader2, CheckCircle, Lock, Send, RotateCcw, Zap } from 'lucide-react';
 import type { MarketplaceItem } from '@/lib/types';
 
 /**
@@ -41,11 +43,29 @@ export function ListingTradeActions({ item }: { item: MarketplaceItem }) {
   const escrow = useCashuEscrow();
 
   const [showEscrow, setShowEscrow] = useState(false);
+  const [showPay, setShowPay] = useState(false);
 
   const isOwnListing = user?.pubkey === item.pubkey;
   const isSold = item.status === 'sold';
   const isWTB = item.listingType === 'wtb';
 
+  // "Buy with sats" for fixed-price listings: resolve the price to sats — a SATS
+  // price directly, a USD price via the live rate. Other currencies fall back to
+  // the offer/contact flow.
+  const priceCurrency = item.price?.currency?.toUpperCase();
+  const priceAmount = Number(item.price?.amount);
+  const usdSats = useUsdToSats(
+    priceCurrency === 'USD' && Number.isFinite(priceAmount) ? priceAmount : null,
+  );
+  const buyAmountSats =
+    item.priceType === 'fixed' && item.price && Number.isFinite(priceAmount)
+      ? priceCurrency === 'SATS' || priceCurrency === 'SAT'
+        ? priceAmount
+        : priceCurrency === 'USD'
+          ? usdSats
+          : null
+      : null;
+
   const sellerNamedMe =
     !!user &&
     !isOwnListing &&
@@ -56,6 +76,16 @@ export function ListingTradeActions({ item }: { item: MarketplaceItem }) {
   const iConfirmedTrade = !!trade?.iConfirmed;
   const iRequestedConfirmation = !!requestState?.iRequested;
 
+  const canBuy =
+    !!user &&
+    !isOwnListing &&
+    !isWTB &&
+    !isSold &&
+    !iConfirmedTrade &&
+    item.priceType === 'fixed' &&
+    typeof buyAmountSats === 'number' &&
+    buyAmountSats > 0;
+
   const sellerEscrowPubkey =
     typeof (metadata as Record<string, unknown> | undefined)?.cashu_escrow === 'string'
       ? ((metadata as Record<string, unknown>).cashu_escrow as string)
@@ -194,6 +224,18 @@ export function ListingTradeActions({ item }: { item: MarketplaceItem }) {
         </Button>
       )}
 
+      {/* Buyer, fixed price: pay directly — no negotiation needed */}
+      {canBuy && (
+        <Button
+          size="sm"
+          onClick={() => setShowPay(true)}
+          className="w-full gap-2 bg-[#ccff00] font-bold text-black hover:bg-[#ccff00]/90"
+        >
+          <Zap className="h-4 w-4" />
+          Buy with sats{buyAmountSats ? ` · ${buyAmountSats.toLocaleString()}` : ''}
+        </Button>
+      )}
+
       {/* Contact button */}
       <Button
         variant="outline"
@@ -209,6 +251,22 @@ export function ListingTradeActions({ item }: { item: MarketplaceItem }) {
         {isWTB ? 'I Have This!' : 'Contact Seller'}
       </Button>
 
+      {/* Mounted only while open — keeps the wallet hooks off every grid card. */}
+      {showPay && (
+        <PayWithSats
+          open
+          onOpenChange={(open) => { if (!open) setShowPay(false); }}
+          sellerPubkey={item.pubkey}
+          amountSats={buyAmountSats ?? undefined}
+          onPaid={() =>
+            toast({
+              title: 'Paid ⚡',
+              description: 'Ask the seller to confirm the trade so it counts for both of you.',
+            })
+          }
+        />
+      )}
+
       {canEscrow && sellerEscrowPubkey && (
         <EscrowRequestDialog
           open={showEscrow}