PlayerTracker

新增：

• 新增 /track <玩家名> 指令，允许直接锁定任意玩家，无需再多次切换目标。
• 新增 /untrack 指令，可快速取消当前追踪状态。
• 当目标离开范围时，仍显示其玩家名，不再只显示“未找到目标”， 而不是清除数据。
• 将客户端数据储存在 player_tracker.json 中，支持同时保存 UUID 与玩家名称（离线登录、改名等情况依旧有效）。

Added:

• Added /track <playername> command, to directly lock onto any player by name.
• Added /untrack command to cancel the current tracking target.
• When the target is out of range, still display their player name instead of just showing 'not found', without clearing the data.
• Client data in the player_tracker.json, now stores both UUID and player name — improves compatibility with offline mode and renamed players.