Fix ref typecheck error

In `Ref<T>` the `current` property is now a readonly field. The correct
type (`MutableRef<T>`) is inferred automatically, so we don't need to
specify it.

src/sidebar/components/SidebarView.js

@@ -83,7 +83,6 @@ function SidebarView({
     !hasDirectLinkedAnnotationError &&
     !isLoading;
 
-  /** @type {import("preact/hooks").Ref<string|null>} */
   const prevGroupId = useRef(focusedGroupId);
 
   // Reload annotations when group, user or document search URIs change