fix(17): WR-01 keep a menu entry's subtree and parent in its menu

- The admin refuses a parent from another menu than the one the form
  names (errors.parent_menu) instead of moving the entry to the parent's
  menu without a word; BeforeSave records the named menu
  (MenuEntry.RequestedMenuID).
- An entry moved to another menu moves its descendants in the same
  transaction, so the public tree no longer drops them.

controllers/menuentries.go

@@ -24,6 +24,7 @@ const (
 	errTypeUnknown  = "golem15.menu::lang.errors.type_unknown"
 	errMenuRequired = "golem15.menu::lang.errors.menu_required"
 	errParentCycle  = "golem15.menu::lang.errors.parent_cycle"
+	errParentMenu   = "golem15.menu::lang.errors.parent_menu"
 	labelTypeURL    = "golem15.menu::lang.models.menuentry.type_url"
 	labelTypeDivide = "golem15.menu::lang.models.menuentry.type_divider"
 )
@@ -183,11 +184,33 @@ func (menuEntriesAdminController) FormAfterCreate(ctx context.Context, model any
 }
 
 // FormAfterUpdate also refuses a parent that is the entry itself or one of
-// its descendants.
+// its descendants, then moves the entry's descendants into its menu, so a
+// subtree follows its root to another menu instead of staying behind with
+// parents in a menu it is not in.
 func (menuEntriesAdminController) FormAfterUpdate(ctx context.Context, model any) error {
-	return checkPlacement(ctx, model)
+	if err := checkPlacement(ctx, model); err != nil {
+		return err
+	}
+	entry, ok := model.(*models.MenuEntry)
+	if !ok || entry == nil || entry.ID == 0 {
+		return nil
+	}
+	tx, ok := cabana.TxFromContext(ctx)
+	if !ok {
+		return errors.New("golem15.menu: the entry check needs the save's transaction")
+	}
+	return tx.Session(&gorm.Session{NewDB: true}).WithContext(ctx).Exec(`WITH RECURSIVE sub(id) AS (
+	SELECT id FROM golem15_menu_menu_entries WHERE parent_id = ? AND id <> ?
+	UNION
+	SELECT e.id FROM golem15_menu_menu_entries e JOIN sub ON e.parent_id = sub.id
+)
+UPDATE golem15_menu_menu_entries SET menu_id = ? WHERE id IN (SELECT id FROM sub) AND id <> ? AND menu_id <> ?`,
+		entry.ID, entry.ID, entry.MenuID, entry.ID, entry.MenuID).Error
 }
 
+// checkPlacement refuses an entry in no menu, a parent from another menu
+// than the one the entry names (BeforeSave would otherwise move the entry to
+// the parent's menu without a word) and a parent cycle.
 func checkPlacement(ctx context.Context, model any) error {
 	entry, ok := model.(*models.MenuEntry)
 	if !ok || entry == nil {
@@ -196,6 +219,9 @@ func checkPlacement(ctx context.Context, model any) error {
 	if entry.MenuID == 0 {
 		return &cabana.ValidationError{Details: map[string]any{"menu": []string{errMenuRequired}}}
 	}
+	if entry.RequestedMenuID() != 0 {
+		return &cabana.ValidationError{Details: map[string]any{"parent": []string{errParentMenu}}}
+	}
 	if entry.ParentID == nil {
 		return nil
 	}