fix(dashboard): Return default preferences instead of 404

The GetPreferences API was returning 404 when no preferences existed
for a library, breaking the dashboard settings modal. Now returns
default preferences (empty hidden_collections, empty collection_order,
20 items_per_section) when no preferences are found, matching the
behavior of the frontend dashboard page.

internal/handlers/dashboard.go

@@ -4,6 +4,7 @@ import (
 	"bookhoard/internal/database"
 	"bookhoard/internal/services"
 	"bookhoard/internal/utils"
+	"log"
 	"net/http"
 	"strconv"
 
@@ -200,7 +201,6 @@ func (h *DashboardHandler) GetPreferences(c echo.Context) error {
 	userUUID := uuid.UUID(user.ID.Bytes)
 	libraryID := c.QueryParam("library_id")
 
-	// ✅ Add validation
 	if libraryID == "" {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "library_id required"})
 	}
@@ -211,7 +211,13 @@ func (h *DashboardHandler) GetPreferences(c echo.Context) error {
 	}
 	prefs, err := h.dashboardService.GetDashboardPreferences(c.Request().Context(), userUUID, libUUID)
 	if err != nil {
-		return c.JSON(http.StatusNotFound, map[string]string{"error": "Preferences not found"})
+		// Return default preferences instead of 404 when none exist
+		log.Printf("GetDashboardPreferences failed: %v", err)
+		prefs = database.UserDashboardPreferences{
+			HiddenCollections: []string{},
+			CollectionOrder:   []string{},
+			ItemsPerSection:   pgtype.Int4{Int32: 20, Valid: true},
+		}
 	}
 
 	// Normalize nil slices to empty arrays for JSON