refactor(handlers): relax library_id validation for All Libraries

- dashboard.go: library_id query param is now optional. Empty/missing
  library_id is passed as pgtype.UUID{Valid: false} to the service
  layer, enabling All Libraries mode.

- series.go: library_id is optional for series listing. GetSeriesBooks
  no longer receives a libraryID — it always returns all books in a
  series regardless of library.

- collections.go: Restructure GetCollection to handle system
  collections (query_type != "") with an optional libraryID. When
  libraryID is empty (All Libraries), GetDashboardSections receives
  pgtype.UUID{Valid: false} so no library filter is applied.

internal/handlers/series.go

@@ -9,6 +9,7 @@ import (
 	"strconv"
 
 	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5/pgtype"
 	"github.com/labstack/echo/v5"
 )
 
@@ -24,12 +25,13 @@ func NewSeriesHandler(db *database.Queries) *SeriesHandler {
 
 func (h *SeriesHandler) GetSeries(c *echo.Context) error {
 	libraryID := c.QueryParam("library_id")
-	if libraryID == "" {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "library_id required"})
-	}
-	libUUID, err := uuid.Parse(libraryID)
-	if err != nil {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
+	var libUUID pgtype.UUID
+	if libraryID != "" {
+		parsed, err := uuid.Parse(libraryID)
+		if err != nil {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
+		}
+		libUUID = pgtype.UUID{Bytes: parsed, Valid: true}
 	}
 
 	limit := 20
@@ -81,21 +83,12 @@ func (h *SeriesHandler) GetSeries(c *echo.Context) error {
 }
 
 func (h *SeriesHandler) GetSeriesBooks(c *echo.Context) error {
-	libraryID := c.QueryParam("library_id")
-	if libraryID == "" {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "library_id required"})
-	}
-	libUUID, err := uuid.Parse(libraryID)
-	if err != nil {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
-	}
-
 	seriesName := c.QueryParam("name")
 	if seriesName == "" {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "name required"})
 	}
 
-	books, err := h.seriesService.GetSeriesBooks(c.Request().Context(), libUUID, seriesName)
+	books, err := h.seriesService.GetSeriesBooks(c.Request().Context(), seriesName)
 	if err != nil {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "Failed to load series books"})
 	}
@@ -118,7 +111,7 @@ func (h *SeriesHandler) GetSeriesBooks(c *echo.Context) error {
 	})
 }
 
-func GetSeriesCardsData(ctx context.Context, db *database.Queries, libraryID uuid.UUID, limit, offset int) ([]services.SeriesInfo, int, error) {
+func GetSeriesCardsData(ctx context.Context, db *database.Queries, libraryID pgtype.UUID, limit, offset int) ([]services.SeriesInfo, int, error) {
 	svc := services.NewSeriesService(db)
 	return svc.GetSeriesPage(ctx, libraryID, limit, offset)
 }