fix: resolve nil pubsub preventing live game updates

v.PubSub() was captured at startup before v.Start() initialized NATS,
so both stores held nil and notify() silently no-oped. Replace the
PubSub interface with a callback that evaluates v.PubSub() lazily at
call time.

main.go

@@ -81,8 +81,12 @@ func main() {
 	subFS, _ := fs.Sub(assets, "assets")
 	v.StaticFS("/assets/", subFS)
 
-	store.SetPubSub(v.PubSub())
-	snakeStore.SetPubSub(v.PubSub())
+	store.SetNotifyFunc(func(gameID string) {
+		v.PubSub().Publish("game."+gameID, nil)
+	})
+	snakeStore.SetNotifyFunc(func(gameID string) {
+		v.PubSub().Publish("snake."+gameID, nil)
+	})
 
 	// Home page - tabbed lobby
 	v.Page("/", func(c *via.Context) {