Add sort functionality to /collections/:slug pages with 6 sort options
(featured, newest, price ascending/descending, name A-Z/Z-A). Sort
selection persists across category navigation via URL query params.
Refactor handle_params to be DRY using load_collection/1 helper.
Add comprehensive unit tests for PreviewData category functions and
LiveView tests for the Collection page sorting and navigation.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
