hscontrol: use Updates() instead of Save() for partial updates

Changed UpdateUser and re-registration flows to use Updates() which only
writes modified fields, preventing unintended overwrites of unchanged fields.

Also updated UsePreAuthKey to use Model().Update() for single field updates
and removed unused NodeSave wrapper.

hscontrol/state/state.go

@@ -300,7 +300,9 @@ func (s *State) UpdateUser(userID types.UserID, updateFn func(*types.User) error
 			return nil, err
 		}
 
-		if err := tx.Save(user).Error; err != nil {
+		// Use Updates() to only update modified fields, preserving unchanged values.
+		err = tx.Updates(user).Error
+		if err != nil {
 			return nil, fmt.Errorf("updating user: %w", err)
 		}
 
@@ -1191,9 +1193,10 @@ func (s *State) HandleNodeFromAuthPath(
 			return types.NodeView{}, change.EmptySet, fmt.Errorf("node not found in NodeStore: %d", existingNodeSameUser.ID())
 		}
 
-		// Use the node from UpdateNode to save to database
 		_, err = hsdb.Write(s.db.DB, func(tx *gorm.DB) (*types.Node, error) {
-			if err := tx.Save(updatedNodeView.AsStruct()).Error; err != nil {
+			// Use Updates() to preserve fields not modified by UpdateNode.
+			err := tx.Updates(updatedNodeView.AsStruct()).Error
+			if err != nil {
 				return nil, fmt.Errorf("failed to save node: %w", err)
 			}
 			return nil, nil
@@ -1410,9 +1413,10 @@ func (s *State) HandleNodeFromPreAuthKey(
 			return types.NodeView{}, change.EmptySet, fmt.Errorf("node not found in NodeStore: %d", existingNodeSameUser.ID())
 		}
 
-		// Use the node from UpdateNode to save to database
 		_, err = hsdb.Write(s.db.DB, func(tx *gorm.DB) (*types.Node, error) {
-			if err := tx.Save(updatedNodeView.AsStruct()).Error; err != nil {
+			// Use Updates() to preserve fields not modified by UpdateNode.
+			err := tx.Updates(updatedNodeView.AsStruct()).Error
+			if err != nil {
 				return nil, fmt.Errorf("failed to save node: %w", err)
 			}