Update exchange form error handling to show link to existing trade

frontend/app/exchange/page.tsx

@@ -68,6 +68,7 @@ export default function ExchangePage() {
 
   // UI state
   const [error, setError] = useState<string | null>(null);
+  const [existingTradeId, setExistingTradeId] = useState<number | null>(null);
   const [isBooking, setIsBooking] = useState(false);
 
   // Compute dates
@@ -266,7 +267,17 @@ export default function ExchangePage() {
       // Redirect to trades page after successful booking
       router.push("/trades");
     } catch (err) {
-      setError(err instanceof Error ? err.message : "Failed to book trade");
+      const errorMessage = err instanceof Error ? err.message : "Failed to book trade";
+      setError(errorMessage);
+
+      // Check if it's a "same day" error and extract trade ID
+      const tradeIdMatch = errorMessage.match(/Trade ID: (\d+)/);
+      if (tradeIdMatch) {
+        setExistingTradeId(parseInt(tradeIdMatch[1], 10));
+      } else {
+        setExistingTradeId(null);
+      }
+
       setIsBooking(false);
     }
   };
@@ -297,7 +308,18 @@ export default function ExchangePage() {
         <h1 style={typographyStyles.pageTitle}>Exchange Bitcoin</h1>
         <p style={typographyStyles.pageSubtitle}>Buy or sell Bitcoin with a 5% premium</p>
 
-        {error && <div style={bannerStyles.errorBanner}>{error}</div>}
+        {error && (
+          <div style={bannerStyles.errorBanner}>
+            {error}
+            {existingTradeId && (
+              <div style={styles.errorLink}>
+                <a href={`/trades/${existingTradeId}`} style={styles.errorLinkAnchor}>
+                  View your existing trade →
+                </a>
+              </div>
+            )}
+          </div>
+        )}
 
         {/* Price Display */}
         <div style={styles.priceCard}>
@@ -998,6 +1020,18 @@ const styles: Record<string, CSSProperties> = {
     borderRadius: "6px",
     border: "1px solid rgba(251, 146, 60, 0.2)",
   },
+  errorLink: {
+    marginTop: "0.75rem",
+    paddingTop: "0.75rem",
+    borderTop: "1px solid rgba(255, 255, 255, 0.1)",
+  },
+  errorLinkAnchor: {
+    fontFamily: "'DM Sans', system-ui, sans-serif",
+    color: "#a78bfa",
+    textDecoration: "none",
+    fontWeight: 500,
+    fontSize: "0.9rem",
+  },
   section: {
     marginBottom: "2rem",
   },