feat(series): add SeriesService and wire continue-series into dashboard

Create SeriesService with methods for paginated series listing, cover
path resolution, series book listing, and a conversion helper for
GetContinueSeriesItemsRow to MediaItems.

Wire the continue-series query type into DashboardService's
getCollectionItemsByQueryType switch and add its metadata to the
RestoreSystemCollection default collection map.

internal/services/dashboard_service.go

@@ -292,6 +292,20 @@ func (s *DashboardService) getCollectionItemsByQueryType(ctx context.Context, co
 			LibraryID: pgtype.UUID{Bytes: libraryID, Valid: true},
 			Limit:     int32(limit),
 		})
+	case "continue-series":
+		rows, err := s.db.GetContinueSeriesItems(ctx, database.GetContinueSeriesItemsParams{
+			LibraryID: pgtype.UUID{Bytes: libraryID, Valid: true},
+			UserID:    pgtype.UUID{Bytes: userID, Valid: true},
+			Limit:     int32(limit),
+		})
+		if err != nil {
+			return nil, err
+		}
+		items := make([]database.MediaItems, 0, len(rows))
+		for _, row := range rows {
+			items = append(items, continueSeriesRowToMediaItems(row))
+		}
+		return items, nil
 	default:
 		return []database.MediaItems{}, nil
 	}
@@ -407,6 +421,7 @@ func (s *DashboardService) RestoreSystemCollection(ctx context.Context, userID u
 		"Recently Added":   {"Newly added items to this library", "🆕", "#9ece6a", 2, "recently-added"},
 		"Recently Read":    {"Books you've finished (progress >= 1)", "✅", "#e0af68", 3, "recently-read"},
 		"Not Started":      {"Books you haven't read yet (progress = 0 or no record)", "📕", "#f7768e", 4, "not-started"},
+		"Continue Series":  {"Next book in series you're reading", "📚", "#bb9af7", 5, "continue-series"},
 	}
 
 	meta, exists := defaultMetadata[collectionName]