Add view toggle button

[Aidanpgh]

Description

Description:

Add toggle button in topic list bar (the bar that contains Tracking, All Tags, Recently Replied, and New Topic) to switch between grouped and flat views.

Acceptance Criteria:

1. Button appears next to sort dropdown in category view only
2. Shows list icon for flat view, calendar icon for grouped view
3. Clicking sends API request to update user preference
4. Page reloads with new view after successful update

Community forum reference

No response