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.

internal/router/router.go

@@ -56,6 +56,7 @@ type Config struct {
 	FiltersHandler          *handlers.FiltersHandler
 	DashboardHandler        *handlers.DashboardHandler
 	DashboardService        *services.DashboardService
+	SeriesHandler           *handlers.SeriesHandler
 	OPDSHandler             *handlers.OPDSHandler
 	SystemSettingsHandler   *handlers.SystemSettingsHandler
 	ConnManager             *sync.ConnectionManager
@@ -211,6 +212,7 @@ func RegisterRoutes(cfg *Config) *handlers.Handler {
 	registerSystemRoutes(cfg)
 	registerSyncRoutes(cfg)
 	registerCollectionsRoutes(cfg)
+	registerSeriesRoutes(cfg)
 	registerDashboardRoutes(cfg)
 	registerMediaRoutes(cfg)
 	registerSearchRoutes(cfg)