V124 - ACCOUNT CHARACTER FILE MOVE (NO FOLDER RENAME)

Observation confirmed in BG2: if the normal `characters` directory is renamed or missing,
BG2 recreates it. Therefore the directory itself is a required engine working folder and is
not a reliable ownership container.

V124 keeps `characters` permanently present.

SAVE ACCOUNT CHARACTER:
- temporarily isolates loose non-account CHR/BIO files by moving them to a local hold folder;
- lets BG2 export normally into `characters`;
- moves only the exported <name>.CHR and matching <name>.BIO into characters_<account>;
- restores any isolated loose files;
- overwrites only the same account's same character filename on later saves.

LOAD ACCOUNT CHARACTER:
- temporarily isolates loose non-account CHR/BIO files;
- moves that account's CHR/BIO files from characters_<account> into `characters`;
- opens BG2's native import list;
- on Done or Cancel, moves those account files back and restores loose files.

There are no character backup copies, automatic restores, merges, synchronization or respawns.
The helper is a local filesystem mover only; it is not an account/network server.
