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/useNavigatorStore.test.tsx

@@ -8,7 +8,7 @@ describe('navigator filter shapes (smoke)', () =>
     {
         const { result } = renderHook(() => useNavigatorData());
         expect(Object.keys(result.current).sort()).toEqual([
-            'categories', 'eventCategories', 'favouriteRoomIds',
+            'categories', 'eventCategories',
             'navigatorData', 'navigatorSearches',
             'topLevelContext', 'topLevelContexts'
         ].sort());