Fix artifact API - use correct parameter and field names

Correct the artifact implementation to match Unsandbox API:
- Request parameter: return_artifact (boolean, singular)
- Response field: artifacts (array, plural)
- Data field: content_base64 (not data/content)

Changes:
- Backend: Changed artifacts to return_artifact in request body
- Frontend: Use return_artifact in execute request
- Frontend: Look for artifacts array in response (not artifact singleton)
- Frontend: Decode content_base64 field (not data/content)
- Frontend: Use filename field (primary) over name
- Remove debug console.log statements
- Update CLAUDE.md with correct API contract

The API parameter is singular but response is plural - this is intentional
design by Unsandbox API. Request enables artifact collection, response
returns array of generated artifacts.

app.py

@@ -1276,7 +1276,7 @@ def proxy_code_execute():
         request_body = {
             "language": language,
             "code": code,
-            "return_artifacts": True,
+            "return_artifact": True,
         }
 
         # Use SDK's internal _make_request for full parameter support