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/handlers/auth.go

@@ -1015,6 +1015,7 @@ func (h *AuthHandler) CreateDefaultCollectionsForUser(ctx context.Context, userI
 		{"Recently Added", "Newly added items to this library", "🆕", "#9ece6a", "recently-added", 2},
 		{"Recently Read", "Books you've finished (progress >= 1)", "✅", "#e0af68", "recently-read", 3},
 		{"Not Started", "Books you haven't read yet (progress = 0 or no record)", "📕", "#f7768e", "not-started", 4},
+		{"Continue Series", "Next book in series you're reading", "📚", "#bb9af7", "continue-series", 5},
 	}
 
 	for _, col := range defaultCollections {