feat(navigator): reactive favourites via fine-grained store (P3)

Move favourite room ids out of the useBetween navigator store into a
dedicated Zustand store. useNavigatorFavourite(roomId) subscribes only
to s.ids.has(roomId) (a boolean), so a FavouriteChangedEvent for one
room no longer re-renders every favourite-aware view. apply() returns
the same state reference when membership is unchanged.

src/hooks/navigator/index.ts

@@ -1,4 +1,5 @@
 export { useNavigatorData } from './useNavigatorData';
+export { useNavigatorFavourite } from './useNavigatorFavourite';
 export { useNavigatorSearch } from './useNavigatorSearch';
 export { useNavigatorUiState } from './useNavigatorUiState';
 export { useNavigatorUiStore } from './navigatorUiStore';