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/cloud_api_client/src/cloud_api_client.rs

@@ -35,6 +35,10 @@ impl CloudApiClient {
         });
     }
 
+    pub fn clear_credentials(&self) {
+        *self.credentials.write() = None;
+    }
+
     fn authorization_header(&self) -> Result<String> {
         let guard = self.credentials.read();
         let credentials = guard