Display return values and fix duplicate error messages in REPL

- javascript-repl now displays return values with => prefix
- Objects are JSON.stringify'd with formatting
- Removed console.error from wrapper to prevent duplicate error messages
- Error is already captured and sent via execution-error message
- Fixes missing return value display and duplicate error output

packages/web-ui/src/components/SandboxedIframe.ts

@@ -347,8 +347,7 @@ export class SandboxIframe extends LitElement {
 
 				await window.complete(null, returnValue);
 			} catch (error) {
-				console.error(error?.stack || error?.message || String(error));
-
+	
 				// Call completion callbacks before complete() (error path)
 				if (window.__completionCallbacks && window.__completionCallbacks.length > 0) {
 					try {

packages/web-ui/src/tools/javascript-repl.ts

@@ -63,6 +63,12 @@ export async function executeJavaScript(
 			}
 		}
 
+		// Add return value if present
+		if (result.returnValue !== undefined) {
+			if (output) output += "\n";
+			output += `=> ${typeof result.returnValue === "object" ? JSON.stringify(result.returnValue, null, 2) : result.returnValue}`;
+		}
+
 		// Add file notifications
 		if (result.files && result.files.length > 0) {
 			output += `\n[Files returned: ${result.files.length}]\n`;