Integrate BTCPay Server: auto-settling invoice deposits

- lib/btcpay.ts: Greenfield API client (createBtcPayInvoice, getBtcPayInvoiceStatus)
- /api/btcpay/invoice: creates per-deposit invoice (USD amount + handle in metadata)
- /api/btcpay/status/[id]: polls invoice status + returns BTC address/amount
- add-funds page: BTCPay tab with amount picker, live address display,
  10s auto-poll, settlement auto-credits localStorage ledger
- WalletContext: creditBtcPayInvoice() with duplicate-invoice guard
- .env.example: BTCPAY_URL, BTCPAY_API_KEY, BTCPAY_STORE_ID, NEXT_PUBLIC_BTCPAY_ENABLED

Made-with: Cursor

contexts/WalletContext.tsx

@@ -13,6 +13,7 @@ interface WalletContextType {
   earnLuxCredits: (amount: number) => void;
   spendUsdStoreCredit: (amount: number) => boolean;
   verifyBtcDeposit: (txid: string) => Promise<{ ok: boolean; error?: string; creditedUsd?: number }>;
+  creditBtcPayInvoice: (invoiceId: string, usdAmount: number) => { ok: boolean; error?: string };
   vault: VaultState;
   collectKey: (key: string) => boolean;
   addVaultReceipt: (receipt: VaultReceipt) => void;
@@ -141,6 +142,23 @@ export const WalletProvider = ({ children }: WalletProviderProps) => {
     [handle],
   );
 
+  const creditBtcPayInvoice = useCallback(
+    (invoiceId: string, usdAmount: number) => {
+      if (!handle) return { ok: false, error: "Sign in first" };
+      const id = String(invoiceId || "").trim();
+      if (!id) return { ok: false, error: "Invalid invoice ID" };
+      const safe = Number.isFinite(usdAmount) ? Math.max(0, Math.round(usdAmount * 100) / 100) : 0;
+      if (safe <= 0) return { ok: false, error: "Invalid amount" };
+      const row = getLedgerRow(handle);
+      if (row.claimedTxids.includes(id)) return { ok: false, error: "Invoice already credited" };
+      const nextUsd = Math.round((row.usd + safe) * 100) / 100;
+      setLedgerRow(handle, { usd: nextUsd, lux: row.lux, claimedTxids: [...row.claimedTxids, id] });
+      setUsdStoreCredit(nextUsd);
+      return { ok: true };
+    },
+    [handle],
+  );
+
   const collectKey = (key: string) => {
     const trimmed = String(key || "").trim();
     if (!trimmed) return false;
@@ -167,6 +185,7 @@ export const WalletProvider = ({ children }: WalletProviderProps) => {
       earnLuxCredits,
       spendUsdStoreCredit,
       verifyBtcDeposit,
+      creditBtcPayInvoice,
       vault,
       collectKey,
       addVaultReceipt,
@@ -179,6 +198,7 @@ export const WalletProvider = ({ children }: WalletProviderProps) => {
       earnLuxCredits,
       spendUsdStoreCredit,
       verifyBtcDeposit,
+      creditBtcPayInvoice,
       vault,
     ],
   );