Polish sidebar header appearance

Summary:
Polish the category headers in response to user feedback and for aesthetic reasons.
- Category page: Made the separator line always visible
- Subcategory page: Removed the redundant border lines in the back button and made the separator always visible
- Widescreen view: Made the header text bold to match other headers, and made the background gray match the search bar header that's right next to it

BUG: 392340

Test Plan: {F5771156}

Reviewers: mart, #plasma, #vdg

Reviewed By: mart, #plasma, #vdg

Subscribers: davidedmundson, abetts, richardbowen, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11757