V312 - MAIN MENU BACK + MULTIPLAYER POPUP CLEANUP

INSTALL: Close BG2 completely, extract the package and run SETUP_IN_GAME_ACCOUNTS.bat.
There is no separate V312 installer. Launch normal BaldurII.exe.

CHANGES
- "Retour" in the new-game/Join choice goes to the main game UI instead of closing one popup.
- "Retour" from the create-session screen does the same.
- Once multiplayer gameplay is active, stale connection/setup panels from joining or creating a game are removed; normal gameplay windows, character creation, Import/Export, and the arbitration screen are not suppressed.
- Existing LAN/IPv4/session name changes from V311 remain.
- No account/character router code or character folders changed from V311.
- Engine pause/unpause and real gameplay dialog controls are intentionally unchanged.

Known limit: BG2 2.7.3 on Windows is needed to verify the visual behavior; successful static tests do not confirm engine behavior.
