feat: adopt IINA-style playback window

Sources/Core/player.cpp

@@ -748,6 +748,16 @@ bool player_seek(Player *player, double seconds) {
 bool player_close(Player *player) {
     return submit(player, [=] { return player->enqueue({Action::Close, {}}); });
 }
+bool player_set_volume(Player *player, float normalized0to1) {
+    if (!std::isfinite(normalized0to1) || normalized0to1 < 0 || normalized0to1 > 1) return false;
+    return submit(player, [=] {
+        player->output.set_volume(normalized0to1);
+        return true;
+    });
+}
+float player_get_volume(Player *player) {
+    return player ? player->output.volume() : 0;
+}
 PlayerState player_get_state(Player *player) {
     try {
         if (player) {