DRAKHYSYS V171 - NATIVE ACCOUNT-MIRROR EXPORT: RENAME / OVERWRITE - NO BACKUPS

- Keeps the working V170 account-folder mirror and native Export list.
- Export filename field is editable again.
- Select an existing account export and press Done to use BG2's normal overwrite flow.
- Or type another valid filename and press Done to save another export in the same active account folder.
- The active `characters` junction remains the ownership boundary, so another account's character files are neither listed nor writable.
- Import behavior is unchanged.
- Logout still deactivates/disconnects the active account.
- No automatic character backup/restore logic is added.
