feat: Implement sub-room functionality

- Added support for sub-rooms in the SpaceHierarchy component, allowing rooms to have nested child rooms.
- Introduced UnjoinedSubRoomItem component to display unjoined sub-rooms with a join button.
- Created SubRooms settings page for managing sub-rooms, including adding and removing sub-rooms.
- Updated RoomNavItem to include options for creating sub-rooms.
- Enhanced room fetching logic to filter out sub-rooms from the main room list.
- Added hooks for managing sub-rooms state and permissions.
- Updated relevant state management to handle sub-room creation and association with parent rooms.

src/app/hooks/useRoomSubRooms.ts

@@ -0,0 +1,29 @@
+import { Room } from 'matrix-js-sdk';
+import { useMemo } from 'react';
+import { useStateEvent } from './useStateEvent';
+import { StateEvent } from '../../types/matrix/room';
+
+/**
+ * Content structure for Paarrot sub-rooms
+ */
+export interface PaarrotSubRoomsContent {
+  children: string[];
+}
+
+/**
+ * Hook to get the sub-rooms (child rooms) for a given room.
+ * Sub-rooms are stored in the room's state as im.paarrot.sub_rooms
+ * 
+ * @param room - The parent room
+ * @returns Array of child room IDs
+ */
+export const useRoomSubRooms = (room: Room): string[] => {
+  const subRoomsEvent = useStateEvent(room, StateEvent.PaarrotSubRooms);
+  
+  const subRooms = useMemo(() => {
+    const content = subRoomsEvent?.getContent<PaarrotSubRoomsContent>();
+    return content?.children ?? [];
+  }, [subRoomsEvent]);
+  
+  return subRooms;
+};