### Added
- **Party system** — group up with friends via `/party` (`/p` alias)
- `/party invite <player>` sends a clickable `[Accept]` / `[Deny]` chat component (Adventure API, compatible with all supported server versions)
- Invites expire automatically after 60 seconds
- Party admin is the only member who can initiate a `/bw join`; all online members enter together
- `/party leave`, `/party kick`, `/party admin` (transfer), `/party list`
- **`AllowParties` gamerule** — per-arena toggle to permit or block party joins (configurable via `/bw setupGuis`)
- **Party-aware arena selector GUI** — joining through the GUI now enforces party admin/member roles and checks party size against `maxInTeam`

### Fixed
- `ConfigUpdater` NPE on first startup when `CustomItemsNames` section was absent from language files on disk
- `DataManager.save()` NPE in `onDisable()` when startup had failed mid-way (dataManager was null)
- Party members could bypass the party-admin join restriction by clicking an arena in the selector GUI

### Internal
- Lobby location persistence extracted from `NewBedwars.java` into `LobbyData` utility class (`config/data/LobbyData.java`)
- `PartyManager` refactored to use constructor injection (`Plugin`, `Messages`) — no longer depends on `NewBedwars.plugin` static reference
- Added JUnit 5 + Mockito test suite: 54 tests across `PartyTest`, `PartyManagerTest`, `LangMessagesTest`, `LobbyDataTest`, `JoinCommandTest`, `PartyCommandTest`