Create templates/series.templ with:
- Library selector dropdown (sticky, same pattern as dashboard)
- Loading spinner overlay for AJAX library switching
- Series grid with stacked-cascade multi-cover cards
- Empty state when no series found
- Pagination with Previous/Next links
- SeriesCard sub-template linking to filtered bookshelf view
Add 'Series' nav link in header between 'All Books' and 'Collections'.
Make series badge on book detail page clickable, linking to
/bookshelf?series_filter=<name>&sort=series.
Add 'Continue Series' option to restore system collection modal.
