Use the user from the CloudUserStore to drive the user menu (#35375)

This PR updates the user menu in the title bar to base the "signed in"
state on the user in the `CloudUserStore` rather than the `UserStore`.

This makes it possible to be signed-in—at least, as far as the user menu
is concerned—even when disconnected from Collab.

Release Notes:

- N/A

crates/client/src/client.rs

@@ -1463,6 +1463,7 @@ impl Client {
 
     pub async fn sign_out(self: &Arc<Self>, cx: &AsyncApp) {
         self.state.write().credentials = None;
+        self.cloud_client.clear_credentials();
         self.disconnect(cx);
 
         if self.has_credentials(cx).await {