Subtype Switcher goes off screen when a part has many variants

copied from https://github.com/blowfishpro/B9PartSwitch/issues/199

On a part with 32 variants (albeit at a small screen resolution) the subtype switcher UI goes off screen. It should probably get a scroll bar when there are a lot of variants

<img width="2272" height="1804" alt="Image" src="https://github.com/user-attachments/assets/518f7f2e-0178-46d1-adaa-922e73dbd226" />