V170 - Native account-mirror Export + account-bound write guard - no character backups

Fixes V169 empty Export list / disabled filename regression.

- Restores BG2's stock CHARACTER_EXPORT list table and native list selection action.
- Read visibility is enforced by the active characters -> characters_<account> junction, matching the working Import behavior.
- Existing CHR files physically inside characters_<account> are bootstrapped as owned/readable by that account flag.
- Other account folders are never mounted/readable while this account is active.
- The current player character gets one bound export filename. First-time export allocates it if needed; later exports replace that same file.
- The export write action refuses a filename that is not the current character's bound account id.
- Import Saved Game remains disabled.
- No automatic character backups, backup copies, or deleted-character restoration are added.
