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/navigatorFavouritesStore.ts

@@ -0,0 +1,25 @@
+import { createNitroStore } from '../../state/createNitroStore';
+
+export type NavigatorFavouritesState = {
+    ids: Set<number>;
+};
+
+export type NavigatorFavouritesActions = {
+    setAll(roomIds: number[]): void;
+    apply(roomId: number, added: boolean): void;
+};
+
+export const useNavigatorFavouritesStore = createNitroStore<NavigatorFavouritesState & NavigatorFavouritesActions>()((set) => ({
+    ids: new Set<number>(),
+
+    setAll: (roomIds) => set({ ids: new Set(roomIds.map(Number)) }),
+    apply: (roomId, added) => set((s) =>
+    {
+        const id = Number(roomId);
+        if(added ? s.ids.has(id) : !s.ids.has(id)) return s;
+        const ids = new Set(s.ids);
+        if(added) ids.add(id);
+        else ids.delete(id);
+        return { ids };
+    })
+}));