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 {