Replace the single-purpose SeriesDetail template with a parameterized
BrowseDetail component that accepts badge icon/label, title, page title,
back URL/label, empty state icon/message, and book list. Both series
detail and new tag detail pages use the same template with different
params, eliminating duplication.
Series detail: 📚 Series, back to /series, "All Series"
Tag detail: 🏷️ Tag, back to /bookshelf, "Bookshelf"
Deleted series_detail.templ and series_detail_templ.go.
Updated frontend.go series route to call BrowseDetail with series params.
Added /tags/detail route calling BrowseDetail with tag params.