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.

templates/types.go

@@ -39,6 +39,13 @@ type LibraryData struct {
 	TypeName    string
 }
 
+type SeriesCardData struct {
+	Name          string
+	BookCount     int64
+	TotalInSeries int
+	CoverPaths    []string
+}
+
 type PendingRegistrationData struct {
 	RegistrationID string
 	DeviceName     string