V204 - exact per-account login flag + strict account-folder selector

Base: V203 / V202 / V195 character system.

Login flag rule:
- LOGIN accountname -> set only that account's '<ACCOUNTNAME> Activate' flag to 1.
- If that same account's flag is already 1, that account name is disabled for another login.
- EXIT or LOG OUT -> set only that same account's flag to 0.
- One account's flag never clears another account's flag.

Character account rule:
- accountname=kaos selects characters_kaos.
- Every characters_<accountname> folder contains _DRAKHYSYS_ACCOUNT_FLAG.json.
- The embedded account/flag value must exactly match the folder/account name.
- LOGIN / Import / Export refuse a missing or mismatched account folder and do not recreate it.
- CREATE ACCOUNT remains the only path allowed to create a new account folder/identity flag.

Logout rule:
- LOG OUT sends the old account name to the router.
- The router deactivates exactly that account.
- The shared characters selector is disconnected only if that same account currently owns it.
- BG2 clears the logged-in session only after the router confirms the matching account was disconnected.

Preserved from V202:
- Delete Account remains disabled.
- Stock Abdel/Daria/Lessa/Rothgar Import rows remain hidden.
- Existing normal Import/Export UI behavior remains.
- No automatic character backups or restoration.
