Replaces the `group_by`, `sub_group_by`, and `sort_by` parameters with a single `view` parameter in the digest configuration.
The new `view` parameter can be set to one of three values:
- `date`: Groups by category, then by date.
- `category`: Groups by category, then by feed.
- `ai`: Groups by category, then uses the LLM to create sub-groups.
The default view is `date`.
This change simplifies the configuration for the user and makes the digest generation logic easier to maintain.