V203 - V202 account-character junction self-repair

Base: V202 / V195 character system.

Only the account mirror/junction reliability path is changed:
- Windows broken/stale `characters` junctions are detected even when their target no longer exists.
- A stale junction is removed safely.
- `_DRAKHYSYS_BASE_CHARACTERS` is restored when present before a new account mirror is mounted.
- On login/import/export, `characters` is recreated as a valid junction to the active `characters_<accountname>` folder.
- No CHR/BIO file copy, backup, restore, or duplicate history is added.
- Delete Account remains disabled and the four stock import characters remain hidden as in V202.
