Add user preference for flat vs grouped view in user settings

[Aidanpgh]

Description

Description: Store user preference for grouped vs flat topic view in user settings.

Acceptance Criteria:

1. Add categoryGroupedView boolean field to user settings (default: true)
2. API endpoint to update this preference
3. All users default to grouped view
4. User setting choices should persist when the exit the browser or log out and log back in

Community forum reference

No response