Create SeriesHandler with two API endpoints:
- GET /api/series (paginated series list with covers)
- GET /api/series/books (books in a specific series)
Uses query param ?name=X instead of path param to avoid URL encoding
issues with special characters in series names.
Add GetSeriesCardsData helper returning services.SeriesInfo for use
by the SSR route (avoids handlers→templates import cycle).
Register /api/series routes via registerSeriesRoutes in router.
Add /series SSR route in frontend.go with library-scoped pagination
and error handling, matching the dashboard/bookshelf patterns.
Add SeriesHandler to router Config and instantiate in main.go.
Add SeriesCardData type to templates/types.go.
Add Continue Series as the 5th valid system collection in
dashboard handler and auth handler's CreateDefaultCollectionsForUser.