Add payment method selector to exchange form with threshold enforcement

frontend/app/exchange/page.tsx

@@ -19,9 +19,14 @@ type BookableSlot = components["schemas"]["BookableSlot"];
 type AvailableSlotsResponse = components["schemas"]["AvailableSlotsResponse"];
 
 // Constants from shared config
-const { minAdvanceDays: MIN_ADVANCE_DAYS, maxAdvanceDays: MAX_ADVANCE_DAYS } = constants.exchange;
+const {
+  minAdvanceDays: MIN_ADVANCE_DAYS,
+  maxAdvanceDays: MAX_ADVANCE_DAYS,
+  lightningMaxEur: LIGHTNING_MAX_EUR,
+} = constants.exchange;
 
 type Direction = "buy" | "sell";
+type BitcoinTransferMethod = "onchain" | "lightning";
 type WizardStep = "details" | "booking";
 
 /**
@@ -49,6 +54,8 @@ export default function ExchangePage() {
 
   // Trade form state
   const [direction, setDirection] = useState<Direction>("buy");
+  const [bitcoinTransferMethod, setBitcoinTransferMethod] =
+    useState<BitcoinTransferMethod>("onchain");
   const [eurAmount, setEurAmount] = useState<number>(10000); // €100 in cents
 
   // Date/slot selection state
@@ -95,6 +102,18 @@ export default function ExchangePage() {
     return Math.floor(btcAmount * 100_000_000);
   }, [eurAmount, agreedPrice]);
 
+  // Check if Lightning is disabled due to threshold
+  const isLightningDisabled = useMemo(() => {
+    return eurAmount > LIGHTNING_MAX_EUR * 100;
+  }, [eurAmount]);
+
+  // Auto-switch to onchain if Lightning becomes disabled
+  useEffect(() => {
+    if (isLightningDisabled && bitcoinTransferMethod === "lightning") {
+      setBitcoinTransferMethod("onchain");
+    }
+  }, [isLightningDisabled, bitcoinTransferMethod]);
+
   // Fetch price data
   const fetchPrice = useCallback(async () => {
     setIsPriceLoading(true);
@@ -240,6 +259,7 @@ export default function ExchangePage() {
       await api.post<ExchangeResponse>("/api/exchange", {
         slot_start: selectedSlot.start_time,
         direction,
+        bitcoin_transfer_method: bitcoinTransferMethod,
         eur_amount: eurAmount,
       });
 
@@ -356,6 +376,43 @@ export default function ExchangePage() {
               </button>
             </div>
 
+            {/* Payment Method Selector */}
+            <div style={styles.paymentMethodSection}>
+              <div style={styles.paymentMethodLabel}>
+                Payment Method <span style={styles.required}>*</span>
+              </div>
+              <div style={styles.paymentMethodRow}>
+                <button
+                  onClick={() => setBitcoinTransferMethod("onchain")}
+                  disabled={false}
+                  style={{
+                    ...styles.paymentMethodBtn,
+                    ...(bitcoinTransferMethod === "onchain" ? styles.paymentMethodBtnActive : {}),
+                  }}
+                >
+                  <span style={styles.paymentMethodIcon}>🔗</span>
+                  <span>Onchain</span>
+                </button>
+                <button
+                  onClick={() => setBitcoinTransferMethod("lightning")}
+                  disabled={isLightningDisabled}
+                  style={{
+                    ...styles.paymentMethodBtn,
+                    ...(bitcoinTransferMethod === "lightning" ? styles.paymentMethodBtnActive : {}),
+                    ...(isLightningDisabled ? styles.paymentMethodBtnDisabled : {}),
+                  }}
+                >
+                  <span style={styles.paymentMethodIcon}>⚡</span>
+                  <span>Lightning</span>
+                </button>
+              </div>
+              {isLightningDisabled && (
+                <div style={styles.thresholdMessage}>
+                  Lightning payments are only available for amounts up to €{LIGHTNING_MAX_EUR}
+                </div>
+              )}
+            </div>
+
             {/* Amount Section */}
             <div style={styles.amountSection}>
               <div style={styles.amountHeader}>
@@ -445,6 +502,11 @@ export default function ExchangePage() {
                 <span style={styles.satsValue}>
                   <SatsDisplay sats={satsAmount} />
                 </span>
+                <span style={styles.summaryDivider}>•</span>
+                <span style={styles.summaryPaymentMethod}>
+                  {direction === "buy" ? "Receive via " : "Send via "}
+                  {bitcoinTransferMethod === "onchain" ? "Onchain" : "Lightning"}
+                </span>
               </div>
             </div>
 
@@ -559,6 +621,13 @@ export default function ExchangePage() {
                     <span style={styles.confirmLabel}>Rate:</span>
                     <span style={styles.confirmValue}>{formatPrice(agreedPrice)}/BTC</span>
                   </div>
+                  <div style={styles.confirmRow}>
+                    <span style={styles.confirmLabel}>Payment:</span>
+                    <span style={styles.confirmValue}>
+                      {direction === "buy" ? "Receive via " : "Send via "}
+                      {bitcoinTransferMethod === "onchain" ? "Onchain" : "Lightning"}
+                    </span>
+                  </div>
                 </div>
 
                 <div style={styles.buttonRow}>
@@ -869,6 +938,66 @@ const styles: Record<string, CSSProperties> = {
   summaryDivider: {
     color: "rgba(255, 255, 255, 0.3)",
   },
+  summaryPaymentMethod: {
+    fontFamily: "'DM Sans', system-ui, sans-serif",
+    fontSize: "0.875rem",
+    color: "rgba(255, 255, 255, 0.6)",
+  },
+  paymentMethodSection: {
+    marginBottom: "1.5rem",
+  },
+  paymentMethodLabel: {
+    fontFamily: "'DM Sans', system-ui, sans-serif",
+    color: "rgba(255, 255, 255, 0.7)",
+    fontSize: "0.9rem",
+    marginBottom: "0.75rem",
+  },
+  required: {
+    color: "#f87171",
+  },
+  paymentMethodRow: {
+    display: "flex",
+    gap: "0.5rem",
+  },
+  paymentMethodBtn: {
+    flex: 1,
+    fontFamily: "'DM Sans', system-ui, sans-serif",
+    fontSize: "0.95rem",
+    fontWeight: 600,
+    padding: "0.875rem",
+    background: "rgba(255, 255, 255, 0.05)",
+    border: "1px solid rgba(255, 255, 255, 0.1)",
+    borderRadius: "8px",
+    color: "rgba(255, 255, 255, 0.6)",
+    cursor: "pointer",
+    transition: "all 0.2s",
+    display: "flex",
+    alignItems: "center",
+    justifyContent: "center",
+    gap: "0.5rem",
+  },
+  paymentMethodBtnActive: {
+    background: "rgba(167, 139, 250, 0.15)",
+    border: "1px solid #a78bfa",
+    color: "#a78bfa",
+  },
+  paymentMethodBtnDisabled: {
+    opacity: 0.4,
+    cursor: "not-allowed",
+  },
+  paymentMethodIcon: {
+    fontSize: "1.2rem",
+  },
+  thresholdMessage: {
+    fontFamily: "'DM Sans', system-ui, sans-serif",
+    fontSize: "0.75rem",
+    color: "rgba(251, 146, 60, 0.9)",
+    marginTop: "0.5rem",
+    padding: "0.5rem",
+    background: "rgba(251, 146, 60, 0.1)",
+    borderRadius: "6px",
+    border: "1px solid rgba(251, 146, 60, 0.2)",
+  },
   section: {
     marginBottom: "2rem",
   },