#738 ProxyDurableContext missing 'now' property causes isoformat error in IPC apps

ProxyDurableContext.__getattr__ catches all attribute access and returns a method_proxy function. When app code does ctx.now.isoformat(), the 'now' access returns a function instead of datetime. Fix: added explicit @property now on ProxyDurableContext that calls _remote_call('now') and handles type coercion.