V128 - Native import open-button fix (no character backups)

SAVE behavior is unchanged from the proven V125/V127 flow.

IMPORT now stages the logged-in account CHR/BIO files first. The staging response only enables an OPEN IMPORT LIST button; it no longer attempts to change BG2 menus from a polling/text callback.

Clicking OPEN IMPORT LIST runs createCharScreen:OnImportCharacterButtonClick() from an actual user button event and falls back to showing BG2's stock CHARGEN_IMPORT menu if needed.

The stock import Done/Back actions remain untouched.

No character copies, backups, restore history, merges, or automatic respawns are created.
