bookmark: add deep diff comparison for errors$ observable

In `packages/modules/bookmark/src/BookmarkProvider.ts`, the `errors$` getter has:

```ts
public get errors$(): Observable> {
// TODO - add deep diff
return this.#store.select(errorsSelector, deepEqual);
}
```

Investigate replacing the current equality check with a deep-diff comparison to avoid unnecessary emissions when errors are structurally equal but not reference-equal.

Start in packages/modules/bookmark/src/BookmarkProvider.ts at the errors$ getter and inspect how errorsSelector and deepEqual are used. Investigate a deep-diff comparison that prevents emissions for structurally equal BookmarkFlowError arrays; done means equivalent error results no longer emit unnecessarily while actual changes still do.

Written by the indexing model from the issue text.