PvPIndex Battle

Added

• /pvp alias: /pvp now works as an alias for /battle, making it easier for players to discover the command.
• Leaderboard GUI (/battle leaderboard [mode]): paginated inventory GUI showing player skulls ranked by Elo. Each skull displays wins, losses, K/D, streak, and best streak. Supports per-mode filtering (/battle leaderboard sword) or an overall view. Aliases: /battle lb, /battle top. Requires database.enabled: true in config.yml.
• New LeaderboardGui class in platform-paper with 28-entry pages, arrow-based pagination, and skull-per-player layout.
• New lang keys leaderboard.no_data, leaderboard.unknown_mode, leaderboard.error in all bundled language files (en, de, nl, es, pl, zh).