V123 - SAVE/LOAD account character folder rename

- Account creation/login remains entirely in BG2/Baldur.lua.
- No account/network server.
- Physical account folder names use the full safe account name:
    Drakhysys -> characters_drakhysys
    Rulehyr   -> characters_rulehyr
    Yllia     -> characters_yllia
    Yllidia   -> characters_yllidia
- SAVE ACCOUNT CHARACTER:
    1) characters_<account> is temporarily renamed to characters.
    2) BG2 performs its native CHR/BIO export.
    3) characters is immediately renamed back to characters_<account>.
- LOAD ACCOUNT CHARACTER:
    1) characters_<account> is temporarily renamed to characters.
    2) BG2 opens its native import list.
    3) Done/Cancel renames characters back to characters_<account>.
- No copy, backup, restore, merge, synchronization, or deleted-character respawn.
