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/dashboard.go

@@ -133,6 +133,7 @@ func (h *DashboardHandler) RestoreSystemCollection(c *echo.Context) error {
 		"Recently Added":   true,
 		"Recently Read":    true,
 		"Not Started":      true,
+		"Continue Series":  true,
 	}
 	if !validCollections[req.CollectionName] {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "Invalid system collection name"})