Restructure filters with nested categories and improved UX

- Sort buttons moved to header (text link style with underline)
- Page type: non-expandable section with checkboxes
- Categories: expandable with nested activities
  - Selecting category auto-selects all its activities
  - Category count = sum of activity counts
- SVG chevron icons for expand/collapse
- Reorganized categories: hockey → team sports, skateboard → action sports
- Fixed "most relevant" sort to keep original order when no filters active

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: callestenfelt