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/dashboard.go

@@ -30,12 +30,13 @@ func (h *DashboardHandler) GetSections(c *echo.Context) error {
 	userUUID := uuid.UUID(user.ID.Bytes)
 
 	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}
 	}
 
 	prefs, _ := h.dashboardService.GetDashboardPreferences(c.Request().Context(), userUUID, libUUID)
@@ -202,14 +203,15 @@ func (h *DashboardHandler) GetPreferences(c *echo.Context) error {
 	userUUID := uuid.UUID(user.ID.Bytes)
 	libraryID := c.QueryParam("library_id")
 
-	if libraryID == "" {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "library_id required"})
+	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}
 	}
 
-	libUUID, err := uuid.Parse(libraryID)
-	if err != nil {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
-	}
 	prefs, err := h.dashboardService.GetDashboardPreferences(c.Request().Context(), userUUID, libUUID)
 	if err != nil {
 		// Return default preferences instead of 404 when none exist