feat: add handling for sub-room creation by setting parent-child relationships and copying join rules

src/app/features/create-room/CreateRoomModal.tsx

@@ -64,6 +64,26 @@ function CreateRoomModal({ state }: CreateRoomModalProps) {
             parentSubRoomId
           );
 
+          // Set m.space.child on the parent room pointing to this sub-room
+          await mx.sendStateEvent(
+            parentSubRoomId,
+            StateEvent.SpaceChild,
+            { via: viaServers },
+            newRoomId
+          );
+
+          // Copy parent's join_rules to the sub-room
+          const parentJoinRulesEvent = parentRoom.currentState.getStateEvents(StateEvent.RoomJoinRules, '');
+          if (parentJoinRulesEvent) {
+            const joinRulesContent = parentJoinRulesEvent.getContent();
+            await mx.sendStateEvent(
+              newRoomId,
+              StateEvent.RoomJoinRules,
+              joinRulesContent,
+              ''
+            );
+          }
+
           // Add new room to parent's sub-rooms list
           const stateEvent = parentRoom.currentState.getStateEvents(StateEvent.PaarrotSubRooms, '');
           const currentContent = stateEvent?.getContent() as PaarrotSubRoomsContent | undefined;