feat(series): add SeriesHandler, API routes, and SSR browse page

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.

cmd/server/main.go

@@ -87,6 +87,7 @@ func main() {
 	collectionHandler := handlers.NewCollectionHandler(queries, libraryService, connManager)
 	dashboardService := services.NewDashboardService(queries)
 	dashboardHandler := handlers.NewDashboardHandler(queries)
+	seriesHandler := handlers.NewSeriesHandler(queries)
 	filtersHandler := handlers.NewFiltersHandler(queries)
 	mediaHandler := handlers.NewMediaHandler(queries, libraryService, worker)
 	mediaHandler.SetProgressService(progressService)
@@ -149,6 +150,7 @@ func main() {
 		FiltersHandler:          filtersHandler,
 		DashboardHandler:        dashboardHandler,
 		DashboardService:        dashboardService,
+		SeriesHandler:           seriesHandler,
 		OPDSHandler:             opdsHandler,
 		Worker:                  worker,
 		SystemSettingsHandler:   systemSettingsHandler,