Fix vanish-granted flight persisting after reappear, quit and relog

[MiddleXen]

Summary

Fixes a bug where flight granted by vanish could persist after the player was no longer invisible.

Problem

In the current version, flight reset only happens inside showPlayer() when using /sv off:

• only setAllowFlight(false) is called, setFlying(false) is missing
• no flight reset when a player rejoins while no longer vanished
• no flight reset when ReappearOnQuit auto-unvanishes the player on disconnect

Solution

• Added VisibilityChanger.resetVanishFlight() with shared logic
• Call it from showPlayer(), JoinListener (non-vanished join), and QuitListener (auto-reappear on quit)
• Disable both allowFlight and flying, respecting sv.keepfly and gamemode checks

Changed files

• VisibilityChanger.java
• JoinListener.java
• QuitListener.java