UI BACKGROUND OVERRIDE INSTALL FIX

The UI exporter now installs UI.MENU and the selected background MOS together into the active BG2 override folder.

- Full export writes UI.MENU + <BackgroundResRef>.MOS to both the chosen output folder and Active override when they differ.
- SAVE DIRECTLY TO ACTIVE OVERRIDE now also generates/installs the selected background MOS instead of saving UI.MENU alone.
- START replacement mode refuses to export without a valid background image.
- The exporter verifies the generated MOS is 1024x768 and confirms both UI.MENU and the MOS exist together in Active override.
- The PNG/JPG source remains editor input only; BG2 loads the generated MOS resource from override.
